Biomedical subjects

M T Schmolesky

Publications and source records attributed to M T Schmolesky.

5 recordsLinked to original sources

PubMed2000

Abnormal retinotopic organization of the dorsal lateral geniculate nucleus of the tyrosinase-negative albino cat.

Visual defects associated with hypopigmentation have been studied extensively in Siamese and albino cats. Previous research on tyrosinase-negative albino cats has shown that (1) approximately 95% of all nasal and temporal retinal fibers cross at the optic chiasm, and (2) ocular dominance columns normally found in cortex are replaced with hemiretinal domains. In this study, we compared the retinotopic organization of the dorsal lateral geniculate nucleus (LGNd) and visual cortex in albino cats. Extracellular recordings were conducted in the LGNd, area 17, and area 18 of six albino cats. Receptive fields (RFs) were plotted for all sites. We find that, as in albino visual cortex, the albino LGNd contains (1) normal cells with RFs in the visual hemifield contralateral to the recording site (RFc), (2) abnormal cells with RFs in the ipsilateral hemifield (RFi), (3) abnormal cells with dual, mirror-symmetric RFs, one in each hemifield (RFd), and (4) abnormal cells with broad RFs that span the vertical meridian (RFb). Our data indicate that lamina A and lamina A1 consist predominantly of normal RFc and abnormal RFi cells, respectively. The C laminae contain a mixture of RFc, RFi, RFd, and RFb cells. The interlaminar zones contained RFd cells, RFb cells, or both. Thus, the albino LGNd is arranged into hemiretinal and not ocular dominance laminae. Finally, the percentage of normal cells is significantly larger in area 17 (84%) and area 18 (70%) than in the LGNd (46%), suggesting a suppression of abnormal activity in albino cat cortex, which could underlie the existing competence of visual function in albinos.

Albinism, Oculocutaneous↗

PubMed2000

Degradation of stimulus selectivity of visual cortical cells in senescent rhesus monkeys.

Human visual function declines with age. Much of this decline is probably mediated by changes in the central visual pathways. We compared the stimulus selectivity of cells in primary visual cortex (striate cortex or V1) in young adult and very old macaque monkeys using single-neuron in vivo electrophysiology. Our results provide evidence for a significant degradation of orientation and direction selectivity in senescent animals. The decreased selectivity of cells in old animals was accompanied by increased responsiveness to all orientations and directions as well as an increase in spontaneous activity. The decreased selectivities and increased excitability of cells in old animals are consistent with an age-related degeneration of intracortical inhibition. The neural changes described here could underlie declines in visual function during senescence.

Aging↗

PubMed1998

Signal timing across the macaque visual system.

The onset latencies of single-unit responses evoked by flashing visual stimuli were measured in the parvocellular (P) and magnocellular (M) layers of the dorsal lateral geniculate nucleus (LGNd) and in cortical visual areas V1, V2, V3, V4, middle temporal area (MT), medial superior temporal area (MST), and in the frontal eye field (FEF) in individual anesthetized monkeys. Identical procedures were carried out to assess latencies in each area, often in the same monkey, thereby permitting direct comparisons of timing across areas. This study presents the visual flash-evoked latencies for cells in areas where such data are common (V1 and V2), and are therefore a good standard, and also in areas where such data are sparse (LGNd M and P layers, MT, V4) or entirely lacking (V3, MST, and FEF in anesthetized preparation). Visual-evoked onset latencies were, on average, 17 ms shorter in the LGNd M layers than in the LGNd P layers. Visual responses occurred in V1 before any other cortical area. The next wave of activation occurred concurrently in areas V3, MT, MST, and FEF. Visual response latencies in areas V2 and V4 were progressively later and more broadly distributed. These differences in the time course of activation across the dorsal and ventral streams provide important temporal constraints on theories of visual processing.

Animals↗

PubMed1996

The effect of acoustic pulse intensity upon the electrically elicited blink reflex at positive and negative stimulus onset asynchronies.

The present study examined the effects of acoustic pulse intensity and stimulus onset asynchrony (SOA) on the electrically elicited startle reflex response. Subjects were presented with 165 startle eliciting stimuli: 15 control trials with no acoustic pulse, and 5 trials at each pulse intensity (50, 70, and 90 dB) for each SOA (-80, 60, -40, -20, 0, 20, 40, 60, 80, and 100 ms). The results demonstrated R2 magnitude facilitation at negative, simultaneous, and short positive SOAs. R2 facilitation was greatest in the 90 dB condition and least in the 50 dB condition. R1 facilitation at short positive SOAs was greater for more intense acoustic pulses. These data support the notion that R2 facilitation at near-zero SOAs may be the result of combination of pulse induced potentiation of the electrically elicited startle response and temporal summation of the effects of electrical and acoustic stimuli at the facial motor nucleus.

Acoustic Stimulation↗

PubMedDate not supplied

Neural correlates of boundary perception.

The responses of neurons in areas V1 (17) and V2 (18) of anesthetized and paralyzed rhesus monkeys and cats were recorded while presenting a set of computer-generated visual stimuli that varied in pattern, texture, luminance, and contrast. We find that a class of extrastriate cortical cells in cats and monkeys can signal the presence of boundaries regardless of the cue or cues that define the boundaries. These cue-invariant (CI) cells were rare in area V1 but easily found in V2. CI cortical cells responded more strongly to more salient boundaries regardless of the cue defining the boundaries. Many CI cortical cells responded to illusory contours and exhibited the same degree of orientation and direction selectivity when tested with boundaries defined by different cues. These cells have significant computational power inherent in their receptive fields since they were able to generalize across stimuli and integrate multiple cues simultaneously in order to signal boundaries. Cells in higher order cortical areas such as MT (Albright, 1992), MST (Geesaman & Anderson, 1996), and IT (Sary et al., 1993) have previously been reported to respond in a cue invariant fashion. The present results suggest that the ability to respond to boundaries in a cue-invariant manner originates at relatively early stages of cortical processing.

Animals↗