Biomedical subjects

T Allard

Publications and source records attributed to T Allard.

6 recordsLinked to original sources

PubMed1993

Auditory function in Alzheimer's disease.

The pattern of cerebral degeneration in Alzheimer's disease (AD) patients suggests that basic auditory capacities should be normal in AD, whereas progressively higher levels of auditory function should be increasingly impaired. To test this hypothesis, we administered tests of auditory capacities associated with primary auditory cortex (sound localization and perception of complex tones) and auditory association cortex (phoneme discrimination, timbre discrimination, and tonal memory) to 19 mildly to moderately demented AD patients, 21 elderly control subjects (ECS), and 14 young control subjects (YCS). The results showed significant differences between YCS and ECS on phoneme discrimination with synthetic speech and on tonal memory. The AD group differed from the ECS group on sound localization, one measure of synthetic speech discrimination, and timbre discrimination. Performance did not correlate with age, dementia severity, or duration of illness on any test condition. These findings indicate that although AD is accompanied by specific auditory deficits, the increase in neuropathologic change between primary auditory and auditory association cortices is not reflected in an increased impairment of functions that are mediated by these areas. Degraded aural language comprehension, which is characteristic of AD, likely reflects disruption of language processes, rather than dysfunction specific to auditory circuits.

Acoustic Stimulation↗

PubMed1991

Reorganization of somatosensory area 3b representations in adult owl monkeys after digital syndactyly.

1. These experiments were designed to test the hypothesis that temporally correlated afferent input activity plays a lifelong role in the establishment and modification of receptive fields (RFs) and representational topographies in the primary somatosensory cortex of adult monkeys. They were based in part on the finding that adjacent digits of the hand are represented discontinuously in area 3b of the adult owl monkey. If cortical receptive fields and the details of cortical topographic representations are shaped by the weights of the temporal correlations among afferent inputs, then representational discontinuities between digits would be expected to arise because inputs from the skin surfaces of adjacent digits are largely independent in the critical time domain. 2. In the present experiments, the skin of adjacent digits 3 and 4 of the monkey hand was surgically connected to create an artificial syndactyly, or webbed-finger condition. Highly detailed microelectrode maps of the cortical representation of the syndactyl digits were obtained 3-7.5 mo later. This experimental manipulation greatly increased the amount of simultaneous or nearly simultaneous input from the normally separated, now fused, surfaces of adjacent fingers. 3. Cortical maps of the representations of finger surfaces were highly modified from the normal after a several-month-long period of digital fusion. Specifically, the normal discontinuity between the cortical representations of adjacent fingers was abolished. Within a wide cortical zone, RFs were defined that extended across the line of syndactyly onto the surgically joined skin of both fused digits. The representational topography of the fused digits was similar to any normal single digit and was characterized by a continuous progression of partially overlapping RFs. 4. Control observations revealed that these reorganizational changes cannot be accounted for by any changes in cutaneous innervation induced by the surgery. They must arise from representational changes in the central somatosensory system. 5. These findings reveal that cortical maps can be altered in detail in adult monkeys by modifying the distributed temporal structure of afferent inputs. They support the longstanding hypothesis that the temporal coincidence of inputs plays a role in the grouping of input subsets into specific cortical RFs and, consequently, in the shaping of selected effective cortical inputs and representational topographies throughout life.

Animals↗

PubMed1991

Effects of stimulus context on magnitude estimations and category ratings of perceived loudness--the spacing of intensity intervals.

Effects of stimulus context on magnitude estimations and on category ratings were examined for a range of stimulus intensities of a 1-kHz tone. The stimuli were distributed in equal-interval steps of energy so they formed a perceptual cluster of high-intensity tones with a perceptual outlier at the lowest intensity. According to the Invariance Principle, the shape of the response function should not be affected by the distribution of stimulus intensities. However, neither magnitude estimations nor category ratings yielded the linear functions predicted from the Invariance Principle when plotted on log-log axes. Instead, both procedures yielded concave-upward response functions for the group data as well as for the individual data sets of the six subjects. Moreover, unlike previous reports of a nonlinear relationship, we found a linear relationship between magnitude estimations and category ratings. Rather than implying an equivalence of the underlying sensory scales, however, our results may imply subjects used a similar attention strategy for both procedures. We consider some theoretical suggestions, including an attention-band concept, for modification of a multistage stimulus-response (S-R) transformation model.

Adult↗

PubMed1990

Functional reorganization of primary somatosensory cortex in adult owl monkeys after behaviorally controlled tactile stimulation.

1. Multiple microelectrode maps of the hand representation within and across the borders of cortical area 3b were obtained before, immediately after, or several weeks after a period of behaviorally controlled hand use. Owl monkeys were conditioned in a task that produced cutaneous stimulation of a limited sector of skin on the distal phalanges of one or more fingers. 2. Analysis of microelectrode mapping experiment data revealed that 1) stimulated skin surfaces were represented over expanded cortical areas. 2) Most of the cutaneous receptive fields recorded within these expanded cortical representational zones were unusually small. 3) The internal topography of representation of the stimulated and immediately surrounding skin surfaces differed greatly from that recorded in control experiments. Representational discontinuities emerged in this map region, and "hypercolumn" distances in this map sector were grossly abnormal. 4) Borders between the representations of individual digits and digit segments commonly shifted. 5) The functionally defined rostral border of area 3b shifted farther rostralward, manifesting either an expansion of the cutaneous area 3b fingertip representation into cortical field 3a or an emergence of a cutaneous input zone in the caudal aspect of this normally predominantly deep-receptor representational field. 6) Significant lateralward translocations of the borders between the representations of the hand and face were recorded in all cases. 7) The absolute locations--and in some cases the areas or magnifications--of representations of many skin surfaces not directly involved in the trained behavior also changed significantly. However, the most striking areal, positional, and topographic changes were related to the representations of the behaviorally stimulated skin in every studied monkey. 3. These experiments demonstrate that functional cortical remodeling of the S1 koniocortical field, area 3b, results from behavioral manipulations in normal adult owl monkeys. We hypothesize that these studies manifest operation of the basic adaptive cortical process(es) underlying cortical contributions to perception and learning.

Animals↗

PubMed1988

Receptive fields in the body-surface map in adult cortex defined by temporally correlated inputs.

Receptive fields (RFs) obtained at specific cortical sites can be used to define a topographic map of the body surface in adult mammalian somatosensory cortex. This map is not static, and RFs at particular cortical sites can change in size and location throughout adult life. Conversely, the cortical loci at which a given skin surface is represented can shift hundreds of micrometres across the cortex in the koniocortical field, area 3b (refs 1-12). This plasticity suggests that RFs derive not from rigid anatomical connections, but by the selection of a subset of a large number of inputs. We have proposed that inputs are selected on the basis of temporal correlation 11-15. Here we test this idea by altering the correlation of inputs from two adjacent digits on the adult owl monkey hand by surgically connecting the skin surfaces of the two fingers (the formation of syndactyly). This manipulation increases the correlation of inputs from skin surfaces of adjacent fingers. The striking discontinuity between the zones of representation of adjacent digits on the somatosensory cortex disappeared. These results support the hypothesis that the topography of the body-surface map in the adult cortex is influenced by the temporal correlations of afferent inputs.

Animals↗

PubMed1982

Pure word deafness. Analysis of a case with bilateral lesions and a defect at the prephonemic level.

We have demonstrated that the deficit in speech perception in our patient with pure word deafness is secondary to a prephonemic temporal auditory acuity disorder. We delineated the nature of the auditory processing deficit in our patient with bilateral lesions and then demonstrated the presence of a predicted deficit in phonemic discrimination. This pattern is comparable to previous cases with bilateral lesions and distinct from other cases with unilateral lesions. Review of previous reports suggests that there are two distinct types of pure word deafness: type 1, in which the deficit is prephonemic and related to a temporal auditory acuity disorder, and type 2, a form that is independent of a temporal auditory acuity disorder, and has a deficit in linguistic discrimination that does not adhere to a prephonemic pattern (Denes and Semenza, 1975; Saffran et al., 1976). The former has been associated with bilateral temporal lobe lesions (Naeser, 1974; Chocholle et al., 1975), the latter with left unilateral lesions (Denes and Semenza, 1975; Saffran et al., 1976). The first form is an apperceptive disorder, whereas the second represents a higher disorder in phonemic discrimination and may be considered a fragment of Wernicke's aphasia.

Agnosia↗