Biomedical subjects

M B Calford

Publications and source records attributed to M B Calford.

At least 19 recordsLinked to original sources

PubMed2003

Laminar differences in plasticity in area 17 following retinal lesions in kittens or adult cats.

Circumscribed retinal lesions in adult cats result in a reorganization of circuitry in area 17 such that neurons in the lesion projection zone (LPZ) can now be activated, not from their original receptive fields (RFs) but from regions of normal retina adjacent to the lesion ('ectopic' RFs). We have studied this phenomenon further by making circumscribed monocular retinal lesions in 8-week-old kittens and recording responses to visual stimuli of neurons in the LPZ of area 17 when these cats reached adulthood. These responses have been compared with those in adult-lesioned cats either of relatively short postlesion survival (2-24 weeks) or long postlesion survival (3.5-4.5 years). In both kitten-lesioned and adult-lesioned animals most LPZ neurons recorded from the supragranular layers (II and III) not only exhibited new ectopic RFs when stimuli were presented via the lesioned eye but the RF properties (e.g. the sizes of excitatory RFs, orientation and direction selectivities, velocity preferences and upper cut-off velocities) were often indistinguishable from those seen when stimuli were presented via the nonlesioned eye. Similarly, in both kitten-lesioned and adult-lesioned animals, most LPZ neurons recorded from the granular and infragranular layers (IV, V, VI), like those recorded from the supragranular layers, were binocular. However, in adult-lesioned but not in kitten-lesioned animals, the responses and the upper cut-off velocities of LPZ cells recorded from the granular and infragranular layers to stimuli presented via ectopic RFs tended to be, respectively, substantially weaker and lower than those for stimuli presented via the nonlesioned eye. The age-related laminar differences in reorganizational plasticity of cat striate cortex correlate with the lamino-temporal pattern of distribution of N-methyl-d-aspartate glutamate receptors in striate cortex.

Action Potentials↗

PubMed2002

Topographic reorganization in area 18 of adult cats following circumscribed monocular retinal lesions in adolescence.

Circumscribed laser lesions were made in the nasal retinae of one eye in adolescent cats. Ten to sixteen months later, about 80 % of single neurones recorded in the lesion projection zone (LPZ) of contralateral area 18 (parastriate cortex, area V2) were binocular but when stimulated via the lesioned eye had ectopic discharge fields (displaced to normal retina in the vicinity of the lesion). Although the clear majority of binocular cells recorded from the LPZ responded with higher peak discharge rates to stimuli presented via the non-lesioned eye, the orientation and direction selectivities as well as preferred and upper cut-off velocities for stimuli presented through either eye were very similar. Furthermore, the sizes of the ectopic discharge fields of binocular cells recorded from the LPZ were not significantly different from those of their counterparts plotted via the non-lesioned eye. Thus, monocular retinal lesions performed in adolescent cats induce topographic reorganization in the LPZ of area 18. Although a similar reorganization occurs in area 17 (striate cortex, area V1) of cats in which monocular retinal lesions were made either in adulthood or adolescence, in view of the very different velocity response profiles of ectopic discharge fields in areas 17 and those in area 18, it appears that ectopic discharge fields in area 17 are largely independent of excitatory feedback input from area 18.

Algorithms↗

PubMed2002

Dynamic representational plasticity in sensory cortex.

Studies of the effects of peripheral and central lesions, perceptual learning and neurochemical modification on the sensory representations in cortex have had a dramatic effect in alerting neuroscientists and therapists to the reorganizational capacity of the adult brain. An intriguing aspect of some of these investigations, such as partial peripheral denervation, is the short-term expression of these changes. Indeed, in visual cortex, auditory cortex and somatosensory cortex loss of input from a region of the peripheral receptor epithelium (retinal, basilar and cutaneous, respectively) induces rapid expression of ectopic, or expanded, receptive fields of affected neurons and reorganization of topographic maps to fill in the representation of the denervated area. The extent of these changes can, in some cases, match the maximal extents demonstrated with chronic manipulations. The rapidity, and reversibility, of the effects rules out many possible explanations which involve synaptic plasticity and points to a capacity for representational plasticity being inherent in the circuitry of a topographic pathway. Consequently, topographic representations must be considered as manifestations of physiological interaction rather than as anatomical constructs. Interference with this interaction can produce an unmasking of previously inhibited responsiveness. Consideration of the nature of masking inhibition which is consistent with the precision and order of a topographic representation and which has a capacity for rapid plasticity requires, in addition to stimulus-driven inhibition, a source of tonic input from the periphery. Such input, acting locally to provide tonic inhibition, has been directly demonstrated in the somatosensory system and is consistent with results obtained in auditory and visual systems.

Animals↗

PubMed2001

Cortical plasticity revealed by circumscribed retinal lesions or artificial scotomas.

We review the work of others in which the effects of circumscribed, topographically corresponding binocular retinal lesions on the topographic organization of the visual cortex revealed that there is a substantial degree of topographical plasticity in the primary visual cortices of adult cats and macaque monkeys. Despite the evidence indicating that the reorganization of the topographic map in primary visual cortices of adult cats and macaques related to the input from one eye could be suppressed for a long time by inputs related to the other eye, we observed a substantial degree of topographical plasticity in the primary visual cortices of adult cats in which we have made circumscribed monocular retinal lesions. Overall, in both binocularly and monocularly lesioned adult animals, most cells recorded in the cortical projection zone of the retinal lesion (LPZ), several hours, several weeks or several months after placement of the lesions exhibited 'ectopic' excitatory visual receptive fields (RFs) which were displaced to the normal retina in the immediate vicinity of the lesion. The presence of ectopic RFs in cells recorded in the cortical LPZ, combined with the presence of normal cortical representation of the part of the retina in the vicinity of the lesion, indicate a clear expansion of the cortical representation of the part of the retina surrounding the lesion. When stimulated via the ectopic RFs, cortical cells exhibited normal orientation tuning and in the case of animals with monocular lesions, the orientation tuning of binocular cells when stimulated via ectopic RFs appeared to be very similar to that when the cells were stimulated via the RFs in the normal, unlesioned eye. In both binocularly and monocularly lesioned animals, the responses evoked by optimal visual stimuli from the ectopic RFs were substantially weaker than those evoked from their normal counterparts. Similarly, upper velocity limits were significantly lower when visual stimuli were presented via the ectopic RFs. In contrast to cats in which the retinal lesions were made in adulthood, in cats lesioned monocularly in adolescence (8-11 weeks postnatal), both the peak discharge rates and upper velocity limits of responses to photic stimuli presented via the ectopic RFs were very similar to those to stimuli presented via the normal eye. The intracortical mechanism(s) underlying the long-term cortical plasticity revealed by retinal lesions are likely to be closely linked to the mechanism(s) underlying the short-term reversible enlargement of cortical receptive fields observed with artificial scotomas. Furthermore, a similar putative intracortical mechanism(s) appears to underlie psychophysical phenomena observed in studies of retinal scotomas in humans. Overall, the research reviewed here strongly challenges the view that receptive fields of neurons in mammalian visual cortices are 'hard-wired'.

Animals↗

PubMed2001

Spontaneous and stimulus-evoked intrinsic optical signals in primary auditory cortex of the cat.

Spontaneous and tone-evoked changes in light reflectance were recorded from primary auditory cortex (A1) of anesthetized cats (barbiturate induction, ketamine maintenance). Spontaneous 0.1-Hz oscillations of reflectance of 540- and 690-nm light were recorded in quiet. Stimulation with tone pips evoked localized reflectance decreases at 540 nm in 3/10 cats. The distribution of patches "activated" by tones of different frequencies reflected the known tonotopic organization of auditory cortex. Stimulus-evoked reflectance changes at 690 nm were observed in 9/10 cats but lacked stimulus-dependent topography. In two experiments, stimulus-evoked optical signals at 540 nm were compared with multiunit responses to the same stimuli recorded at multiple sites. A significant correlation (P < 0.05) between magnitude of reflectance decrease and multiunit response strength was evident in only one of five stimulus conditions in each experiment. There was no significant correlation when data were pooled across all stimulus conditions in either experiment. In one experiment, the spatial distribution of activated patches, evident in records of spontaneous activity at 540 nm, was similar to that of patches activated by tonal stimuli. These results suggest that local cerebral blood volume changes reflect the gross tonotopic organization of A1 but are not restricted to the sites of spiking neurons.

Acoustic Stimulation↗

PubMed2001

Neurosteroids mediate habituation and tonic inhibition in the auditory midbrain.

Habituation of the behavioral response to a repetitive stimulus is a well-established observation in perceptual studies and is considered a basic form of nonassociative learning. There is also a long history of physiological studies suggesting that central nervous system habituation is mediated by inhibition. At higher levels of the sensory pathways, such inhibition is mainly contributed by GABAa receptor mechanisms. Concepts of modification of synaptic efficacy that apply to excitatory amino acid synaptic transmission do not have direct parallels with these inhibitory synapses: quantal release of GABA rapidly saturates available receptors at a synapse, placing an upper limit on responsiveness to increased transmitter release. However, pharmacological modulation of GABAa-receptor efficacy with exogenous agents (e.g., benzodiazepines and beta-carbolines) is known to occur through allosteric mechanisms that modulate the effectiveness (positive and negative) of GABA at this receptor. The most potent endogenous modulators are 5alpha-reduced steroids. Production of these steroids was attenuated in adult rats with systemic injection of Finasteride, a competitive substrate for 5alpha-reductase. This treatment was sufficient to block habituation of the evoked midbrain response to repetitive presentation of an acoustic click. This result confirms that simple habituation is due to an increase in active inhibition, the increase being mediated by steroid modulation of the GABAa-receptor. Finasteride treatment also brought about a 23% increase in the evoked response to a click stimulus, suggesting that 5alpha-reduced steroids normally contribute to tonic inhibition in the rat inferior colliculus.

3-Oxo-5-alpha-Steroid 4-Dehydrogenase↗

PubMed2000

Plasticity in adult cat visual cortex (area 17) following circumscribed monocular lesions of all retinal layers.

1. In eight adult cats intense, sharply circumscribed, monocular laser lesions were used to remove all cellular layers of the retina. The extents of the retinal lesions were subsequently confirmed with counts of alpha-ganglion cells in retinal whole mounts; in some cases these revealed radial segmental degeneration of ganglion cells distal to the lesion. 2. Two to 24 weeks later, area 17 (striate cortex; V1) was studied electrophysiologically in a standard anaesthetized, paralysed (artificially respired) preparation. Recording single- or multineurone activity revealed extensive topographical reorganization within the lesion projection zone (LPZ). 3. Thus, with stimulation of the lesioned eye, about 75 % of single neurones in the LPZ had 'ectopic' visual discharge fields which were displaced to normal retina in the immediate vicinity of the lesion. 4. The sizes of the ectopic discharge fields were not significantly different from the sizes of the normal discharge fields. Furthermore, binocular cells recorded from the LPZ, when stimulated via their ectopic receptive fields, exhibited orientation tuning and preferred stimulus velocities which were indistinguishable from those found when the cells were stimulated via the normal eye. 5. However, the responses to stimuli presented via ectopic discharge fields were generally weaker (lower peak discharge rates) than those to presentations via normal discharge fields, and were characterized by a lower-than-normal upper velocity limit. 6. Overall, the properties of the ectopic receptive fields indicate that cortical mechanisms rather than a retinal 'periphery' effect underlie the topographic reorganization of area 17 following monocular retinal lesions.

Animals↗

PubMed1999

Monocular focal retinal lesions induce short-term topographic plasticity in adult cat visual cortex.

Electrophysiological recording in primary visual cortex (VI) was performed both prior to and in the hours immediately following the creation of a discrete retinal lesion in one eye with an argon laser. Lesion projection zones (LPZs; 21-64 mm2) were defined in the visual cortex by mapping the extent of the lesion onto the topographic representation in cortex. There was no effect on neuronal responses to the unlesioned eye or on its topographic representation. However, within hours of producing the retinal lesion, receptive fields obtained from stimulation of the lesioned eye were displaced onto areas surrounding the scotoma and were enlarged compared with the corresponding field obtained through the normal eye. The proportion of such responsive recording sites increased during the experiment such that 8-11 hours post-lesion, 56% of recording sites displayed neurons responsive to the lesioned eye. This is an equivalent proportion to that previously reported with long-term recovery (three weeks to three months). Responsive neurons were evident as far as 2.5 mm inside the border of the LPZ. The reorganization of the lesioned eye representation produced binocular disparities as great as 15 degrees, suggesting interactions between sites in VI up to 5.5 mm apart.

Animals↗

PubMed1998

Interhemispheric connections of somatosensory cortex in the flying fox.

The interhemispheric connections of somatosensory cortex in the gray-headed flying fox (Pteropus poliocephalus) were examined. Injections of anatomical tracers were placed into five electrophysiologically identified somatosensory areas: the primary somatosensory area (SI or area 3b), the anterior parietal areas 3a and 1/2, and the lateral somatosensory areas SII (the secondary somatosensory area) and PV (pairetal ventral area). In two animals, the hemisphere opposite to that containing the injection sites was explored electrophysiologically to allow the details of the topography of interconnections to be assessed. Examination of the areal distribution of labeled cell bodies and/or axon terminals in cortex sectioned tangential to the pial surface revealed several consistent findings. First, the density of connections varied as a function of the body part representation injected. For example, the area 3b representation of the trunk and structures of the face are more densely interconnected than the representation of distal body parts (e.g., digit 1, D1). Second, callosal connections appear to be both matched and mismatched to the body part representations injected in the opposite hemisphere. For example, an injection of retrograde tracer into the trunk representation of area 3b revealed connections from the trunk representation in the opposite hemisphere, as well as from shoulder and forelimb/wing representations. Third, the same body part is differentially connected in different fields via the corpus callosum. For example, the D1 representation in area 3b in one hemisphere had no connections with the area 3b D1 representation in the opposite hemisphere, whereas the D1 representation in area 1/2 had relatively dense reciprocal connections with area 1/2 in the opposite hemisphere. Finally, there are callosal projections to fields other than the homotopic, contralateral field. For example, the D1 representation in area 1/2 projects to contralateral area 1/2, and also to area 3b and SII.

Animals↗

PubMed1998

Neuroplasticity and psychiatry.

OBJECTIVE: There is increasing concern that the course of psychiatric disorders may be affected by parameters such as the duration and intensity of symptoms of initial episodes of illness. As this indicates that abnormal function produces long-term changes within the brain, a review of the neuroscience literature regarding neuroplasticity is warranted. METHOD: This article is a selective review, focusing in particular on results obtained from physiological experiments assessing plasticity within the mammalian neocortex. The possible relevance of results to psychiatry is discussed. RESULTS: While the most dramatic examples of neuroplasticity occur during a critical period of neural development, neuroplasticity can also occur in adult neocortex. Neuroplasticity appears to be activity-dependent: synaptic pathways that are intensively used may become strengthened, and conversely, there may be depression of transmission in infrequently used pathways. CONCLUSIONS: Results from neurophysiological experiments lend support to the clinical observation that the intensity and duration of a psychiatric disorder may adversely alter its long-term course. Rapid aggressive treatment may prevent this from occurring. While pharmacotherapy may reduce the duration and severity of symptoms, it may also have an independent, as yet unknown, effect on neuroplasticity.

Adult↗

PubMed1995

Retinal detachment induces massive immediate reorganization in visual cortex.

Large inactive regions of the retina of adult cats were produced by the novel method of inducing monocular retinal detachment. Within a few hours, neurones throughout the detachment projection zone in primary visual cortex (55-136 mm2), including some > 4.5 mm from its boundary, were found to have large receptive fields displaced onto intact retina. The new receptive fields of some neurones represented shifts of up to 9 mm across the retinotopic representation. For these rapid changes to occur pre-existing viable circuits must provide a cortical locus with inputs from a wide extent of the retina. Receptive fields, and the retinotopic map, for stimulation of the other eye were unchanged.

Animals↗

PubMed1995

Responsiveness of cat area 17 after monocular inactivation: limitation of topographic plasticity in adult cortex.

1. Recordings were made from neurones in the splenial sulcus of normal adult cats and adult cats which had one eye inactivated by enucleation or photocoagulation of the optic disc. Two visually responsive regions were observed, corresponding to the peripheral representation of visual area 1 (V1) and the splenial visual area. In normal animals, responses to the ipsilateral eye in V1 were restricted to the medial half of the splenial sulcus, up to 45-50 deg eccentricity. Thus, by inactivating the eye contralateral to the experimental hemisphere, we created a region in V1, 1-2 mm wide, that lacked normal inputs. 2. In contrast to results from previous experiments where lesions were placed in the central retina, neurones in the deprived peripheral representation remained unresponsive to light stimuli for up to 12 h after deactivation of the contralateral eye. 3. In animals that were allowed to recover from the monocular deactivation for periods of 2 days to 16 months, there was rearrangement of the retinotopic maps. Receptive fields in regions of cortex that normally represented the monocular crescent were displaced to the temporal border of the binocular field of vision. However, most neurones in the deprived peripheral representation remained unresponsive to visual stimuli even more than 1 year after treatment. This is also in marked contrast with the extensive reorganization that is observed in the central representation of V1 after restricted retinal lesions. Analysis of the cortical magnification factor demonstrates that the change in visual topography is local, and does not involve an overall centro-peripheral shift of the retinotopic map. 4. Among the neurones that did show displaced receptive fields, the response properties were clearly abnormal. They showed a notable lack of spontaneous activity, low firing rates and rapid habituation to repeated stimulation. 5. The low potential for reorganization of the monocular sector of V1 demonstrates that the capacity for plasticity of mature sensory representations varies with location in cortex. Even relatively small pieces of cortex, such as the monocular crescent representations, may not reorganize completely if certain conditions are not met. These results suggest the existence of natural boundaries that may limit the process of reorganization of sensory representations.

Animals↗

PubMed1995

Phase effects in forward masking of the compound action potential: a comparison of responses to stimulus and distortion frequencies.

When a complex stimulus is presented, new frequencies (distortion products, DPs) are generated within the cochlea. The most intense DPs are lower in frequency than the stimulus tones (primaries). It is not clear whether the relative phase of stimuli is encoded by neural channels tuned to the primaries or by channels tuned to the DPs. We estimated the response of auditory nerve fibres tuned to each of these channels as a function of the relative phase of harmonic stimuli. The compound action potential (CAP) evoked by probes at the primary or distortion frequencies was masked by harmonic 2-tone maskers and cochlear generated DPs. The degree of masking reflected the response to the masker of fibres tuned to the probe. Changes in relative phase of the primaries resulted in a large modulation of the response of fibres tuned to the DPs. Except for a primary frequency ratio of 1:2, the response of fibres tuned to the primaries was only shallowly modulated by changes in relative phase. However, the level of response to the DPs was much lower than the response to the stimulus tones.

Acoustic Stimulation↗

PubMed1995

Monaural inhibition in cat auditory cortex.

1. Several studies of auditory cortex have examined the competitive inhibition that can occur when appropriate sounds are presented to each ear. However, most cortical neurons also show both excitation and inhibition in response to presentation of stimuli at one ear alone. The extent of such inhibition has not been described. Forward masking, in which a variable masking stimulus was followed by a fixed probe stimulus (within the excitatory response area), was used to examine the extent of monaural inhibition for neurons in primary auditory cortex of anesthetized cats (barbiturate or barbiturate-ketamine). Both the masking and probe stimuli were 50-ms tone pips presented to the contralateral ear. Most cortical neurons showed significant forward masking at delays beyond which masking effects in the auditory nerve are relatively small compared with those seen in cortical neurons. Analysis was primarily concerned with such components. Standard rate-level functions were also obtained and were examined for nonmonotonicity, an indication of level-dependent monaural inhibition. 2. Consistent with previous reports, a wide range of frequency tuning properties (excitatory response area shapes) was found in cortical neurons. This was matched by a wide range of forward-masking-derived inhibitory response areas. At the most basic level of analysis, these were classified according to the presence of lateral inhibition, i.e., where a probe tone at a neuron's characteristic frequency was masked by tones outside the limits of the excitatory response area. Lateral inhibition was a property of 38% of the sampled neurons. Such neurons represented 77% of those with nonmonotonic rate-level functions, indicating a strong correlation between the two indexes of monaural inhibition; however, the shapes of forward masking inhibitory response areas did not usually correspond with those required to account for the "tuning" of a neuron. In addition, it was found that level-dependent inhibition was not added to by forward masking inhibition. 3. Analysis of the discharges to individual stimulus pair presentations, under conditions of partial masking, revealed that discharges to the probe occurred independently of discharges to the preceding masker. This indicates that even when the masker is within a neuron's excitatory response area, forward masking is not a postdischarge habituation phenomenon. However, for most neurons the degree of masking summed over multiple stimulus presentations appears determined by the same stimulus parameters that determine the probability of response to the masker.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗

PubMed1994

Level-dependent representation of stimulus frequency in cat primary auditory cortex.

The tonotopicity of the cat's primary auditory cortex (AI) is thought to provide the framework for frequency-specific processing in that field. This study was designed to assess this postulate by examining the spatial distribution of neurons within AI that are activated by a single tonal frequency delivered to the contralateral ear. Distributions obtained at each of several stimulus levels were then compared to assess the influence of stimulus amplitude on the spatial representation of a given stimulus frequency in AI. Data were obtained from 308 single units in AI of four adult, barbiturate-anesthetized cats, using extracellular recording methods. Stimuli were 40-ms tone pulses presented through calibrated, sealed stimulating systems. In each animal, the CF (stimulus frequency to which the unit is most sensitive), threshold at CF, response/level function at CF, and binaural interactions were determined for isolated neurons (usually one per track) in 60-90 electrode tracks. For each unit, regardless of its CF, responses to 40 repetitions of contralateral tones of a single frequency, presented at each of four or five sound pressure levels (SPLs) in the range from 10 to 80 dB were obtained. Different test frequencies were used in each of four cats (1.6, 8.0, 11.0, and 16.0 kHz). For tones of each SPL, we generated maps of the response rates across the cortical surface. These maps were then superimposed on the more traditional maps of threshold CF. All units whose CF was equal to the test frequency could be driven at some SPL, given an appropriate monaural or binaural configuration of the stimulus. There was a clear spatial segregation of neurons according to the shapes of their CF tone response/level functions. Patches of cortex, often occupying more than 2 mm2, seemed to contain only monotonic or only nonmonotonic units. In three cortices, a patch of nonmonotonic cells was bounded ventrally by a patch of monotonic cells, and in one of these cases, a second patch of monotonic cells was found dorsal to the nonmonotonic patch. Contralateral tones of any given SPL evoked excitatory responses in discontinuous cortical territories. At low SPLs (10, 20 dB), small foci of activity occurred along the isofrequency line representing the test frequency. Many of these cells had nonmonotonic response/level functions. (ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1993

Connections of somatosensory cortex in megachiropteran bats: the evolution of cortical fields in mammals.

The cortical connections of the primary somatosensory area (SI or 3b), a caudal somatosensory field (area 1/2), the second somatosensory area (SII), the parietal ventral area (PV), the ventral somatosensory area (VS), and the lateral parietal area (LP) were investigated in grey headed flying foxes by injecting anatomical tracers into electrophysiologically identified locations in these fields. The receptive fields for clusters of neurons were mapped with sufficient density for injection sites to be related to the boundaries of fields, and to representations of specific body parts within the fields. In all cases, cortex was flattened and sectioned parallel to the cortical surface. Sections were stained for myelin and architectonic features of cortex were related to physiological mapping and connection patterns. We found patterns of topographic and nontopographic connections between 3b and adjacent anterior parietal fields 3a and 1/2, and fields caudolateral to 3b (SII and PV). Area 1/2 had both topographic and nontopographic connections with 3b, PP, and SII. Connections of SII and PV with areas 3b, 3a, and 1/2 were roughly topographic, although there was clear evidence for nontopographic connections between these fields. SII was most densely connected with area 1/2, while PV was most densely connected with 3b. SII had additional connections with fields in lateral parietal cortex and with subdivisions of motor cortex. Other connections of PV were with subdivisions of motor cortex and pyriform cortex. Laminar differences in connection patterns of SII and PV with surrounding cortex were also observed. Injections in the ventral somatosensory area revealed connections with SII, PV, area 1/2, auditory cortex, entorhinal cortex, and pyriform cortex. Finally, the lateral parietal field had very dense connections with posterior parietal cortex, caudal temporal cortex, and with subdivisions of motor cortex. Our results indicate that the 3b region is not homogeneous, but is composed of myelin dense and light regions, associated with 3b proper and invaginations of area 1/2, respectively. Connections of myelin dense 3b were different from invaginating portions of myelin light area 1/2. Our findings that 3b is densely interconnected with PV and moderately to lightly interconnected with SII supports the notion that SII and PV have been confused across mammals and across studies. Our connectional evidence provides further support for our hypothesis that area 1/2 is partially incorporated in 3b and has led to theories of the evolution of cortical fields in mammals.

Animals↗

PubMed1993

Rapid changes in the frequency tuning of neurons in cat auditory cortex resulting from pure-tone-induced temporary threshold shift.

The response areas (frequency by intensity) of single neurons in primary auditory cortex of anesthetized cats were studied before and after temporary threshold shifts in cochlear sensitivity induced by an intense pure tone. Cochlear temporary threshold shift was monitored through the threshold of the gross auditory nerve compound action potential and in most cases involved a notch-like loss centered at the characteristic frequency of the unit under study. Only two neurons showed changes in response area that mirrored the changes at the auditory periphery. Most neurons (14) showed more complex changes involving both expansion and contraction of response areas. Expansion of response areas was indicated by lower thresholds at some frequencies and by the emergence of sensitivity to previously ineffective frequencies. A change was classified as contraction when the response area after the intense-tone exposure was smaller than would be expected by applying the profile of the temporary threshold shift to the initial response area. Contraction of both upper (high intensity) and lower boundaries of response areas was found; in the most extreme cases, neurons were totally unresponsive after the intense-tone exposure. The complexity of effects of temporary threshold shifts on the response areas of cortical neurons is likely to be related to mechanisms that normally determine the frequency response limits of these neurons. The response areas of cortical neurons are more complex than those of auditory nerve fibers, and are thought to reflect the integration of excitatory and inhibitory inputs. The variety of effects observed in this study are consistent with the excitatory and inhibitory components of the response area of a given neuron being differentially affected by the temporary threshold shift.

Acoustic Stimulation↗

PubMed1992

Five topographically organized fields in the somatosensory cortex of the flying fox: microelectrode maps, myeloarchitecture, and cortical modules.

Five somatosensory fields were defined in the grey-headed flying fox by using microelectrode mapping procedures. These fields are: the primary somatosensory area, SI or area 3b; a field caudal to area 3b, area 1/2; the second somatosensory area, SII; the parietal ventral area, PV; and the ventral somatosensory area, VS. A large number of closely spaced electrode penetrations recording multiunit activity revealed that each of these fields had a complete somatotopic representation. Microelectrode maps of somatosensory fields were related to architecture in cortex that had been flattened, cut parallel to the cortical surface, and stained for myelin. Receptive field size and some neural properties of individual fields were directly compared. Area 3b was the largest field identified and its topography was similar to that described in many other mammals. Neurons in 3b were highly responsive to cutaneous stimulation of peripheral body parts and had relatively small receptive fields. The myeloarchitecture revealed patches of dense myelination surrounded by thin zones of lightly myelinated cortex. Microelectrode recordings showed that myelin-dense and sparse zones in 3b were related to neurons that responded consistently or habituated to repetitive stimulation respectively. In cortex caudal to 3b, and protruding into 3b, a complete representation of the body surface adjacent to much of the caudal boundary of 3b was defined. Neurons in this area habituated rapidly to repetitive stimulation. We termed this caudal field area 1/2 because it had properties of both area 1 and area 2 of primates. In cortex caudolateral to 3b and lateral to area 1/2 (cortex traditionally defined as SII) we describe three separate representations of the body surface coextensive with distinct myeloarchitectonic appearances. The second somatosensory area, SII, shared a congruent border with 3b at the representation of the nose. In SII, the overall orientation of the body representation was erect. The lips were represented rostrolaterally, the digits were represented laterally, and the toes were caudolateral to the digits. The trunk was represented caudally and the head was represented medially. A second complete representation, PV, had an inverted body representation with respect to SII and bordered SII at the representation of the distal limbs. The proximal body parts were represented rostrolaterally in PV. Finally, caudal to both SII and PV, an additional representation, VS, shared a congruent border with the distal hindlimb representation of both SII and PV. VS had a crude topography, and receptive fields of neurons in VS were relatively large. Many neurons in VS responded to both somatosensory and auditory stimulation.

Animals↗