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C M Bourassa

Publications and source records attributed to C M Bourassa.

At least 19 recordsLinked to original sources
PubMed1998

Differences in the luminance of the first and second displays affects visible persistence in opposite ways.

Visible persistence was measured using a two-frame temporal integration paradigm. Most such studies match the brightness of the two frames, and find that equal increases in the brightness of the frames impairs performance on the task. This suggests that increases in frame brightness decrease the duration of visible persistence. Little is known about what happens when the frames differ in brightness. In this study, the luminance intensities of the first and second frames were set at five different intensity levels in a factorial arrangement. Increasing the intensity of the first frame improved performance, whereas increasing the intensity of the second frame impaired performance. These results suggest, contrary to the findings with brightness-matched frames, that increasing the intensity of one frame increases the duration of visible persistence of that frame. A mathematical model supports this conclusion.

Afterimage↗
PubMed1994

Binocular brightness: a suppression-summation trade off.

In two experiments, 13 and 9 subjects estimated binocular brightness of targets of large visual extent. On each trial one eye was presented with a fairly intense luminance of 800 cd/m2, and the other eye with one of 12 luminances ranging from zero to 800 cd/m2. The first experiment, using ganzfeld stimuli (stimuli of uniform luminance that cover the entire visual field), produced a large amount of binocular brightness summation and very little Fechner's paradox, a decrease in binocular brightness that occurs when the luminances to the two eyes differ greatly. The second experiment, using a smaller target with very low spatial frequencies, produced greater Fechner's paradox than the ganzfelder, but more binocular summation and less Fechner's paradox than what is usually reported for small targets with abrupt contours. The results suggest a trade off between suppressive and summative mechanisms involving binocular cells that are spatially tuned. The trade off is controlled in the vector-sum model by the angle between vectors, which reflects the total inhibition in spatially tuned, binocular channels.

Depth Perception↗
PubMed1991

Sensory neurons of the rat sciatic nerve.

Experiments have been undertaken in this laboratory over recent years to accurately determine the numbers and sizes of somatic neurons which contribute to the normal sciatic nerve, at mid-thigh levels, of the adult, albino rat. This article is concerned with the dorsal root ganglion (DRG) neuron population of the sciatic nerve whose cell bodies were identified through retrograde labeling of cut branches of the sciatic with horseradish peroxidase (HRP) and/or its wheat germ conjugate (WGA-HRP). It is essential to understand the neuronal composition of the normal rat sciatic nerve if the consequences of aging, nerve injury, and surgical repair to improve functional regeneration are to be properly evaluated. Neuron counts were determined from camera-lucida paper drawings of all labeled profiles in DRGs L3-L6 at 100 x magnification. The profiles, obtained by labeling individual branches of the sciatic nerve (sural, lateral sural, tibial, peroneal, medial, and lateral gastrocnemius/soleus nerves) were traced from 40-microns-thick, serial, frozen sections. The sizes of the perikarya, areas and diameters, were determined by tracing the perimeters of the drawn profiles on a digitizing tablet. The tablet's output was inputted directly into a specially designed computer spreadsheet which contained a mathematical table for correcting the split-cell error inherent to the sectioning process. Afferents from any given branch of the sciatic normally occupied two to three adjacent ganglia. Sciatic DRG neurons were normally located in lumbar ganglia L3-L6. Nearly 98-99% of all sciatic DRG perikarya resided in the L4 and L5 DRGs. The L6 DRG, traditionally regarded as an important contributor to the rat sciatic, contained merely 0.4% of its afferent neurons while the L3 ganglion, frequently overlooked as a contributor, contained 1.2% of the mid-thigh sciatic afferents. The mean size of rat DRG neurons was about 29 microns (550-600 microns2). The corrected counts revealed that the normal sciatic nerve (at mid-thigh levels), in rats between 2 and 12 months of age, contained a mean, total DRG neuron population of about 10,500 neurons. This is probably an underestimate by 3-5% of the true number due to occasional unreliable labeling of some of the small DRG neurons. It is estimated that the normal, mean number of sciatic DRG neurons of young to middle-aged rats lies somewhere between 10,500 and 11,000 +/- 2000. The data suggest that nearly 20% of all DRG neurons in the sciatic nerve supply muscle afferents. The vast majority of the remaining neurons are involved with innervation of the skin.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1987

On and off systems in human vision.

Three experiments examined the interaction of On and Off responses that were produced by sudden increments and decrements in luminance. All three experiments utilized a masking technique that required observers to detect a signal in a masking field. The mask was produced by brightening or dimming a field of dots, and the signal consisted of the addition or subtraction of a dot. Experiment 1 showed that detection of the signal-dot was more difficult when the luminance of the signal and the mask changed in the same direction (e.g. a new dot added to the field of dots that were being brightened) than when luminance changed in different directions. When the amplitude of signal and mask was varied parametrically (Experiments 2 and 3), accuracy increased with the ratio of amplitudes of signal and mask. But at any given ratio, the signal was more difficult to detect when signal and mask were of the same sign. The greater difficulty encountered in detecting a signal in the context of a "like" mask is ascribed to greater interference between signal and mask when they share the channel.

Humans↗
PubMed1986

Models for sensation and perception: a selective history.

This paper is a historical survey, for nonspecialists, of models used to account for sensation and perception. Emphasis is placed on several crucial ideas that have given rise to major changes in explanatory models: namely, geometrical optics, receptive fields and systems or computational approaches. The nature of sensory coding and neural representation is briefly considered against this background.

Animals↗
PubMed1985

Equating visibility of brief decrements: unconfounding duration and luminance.

A common procedure in visual psychophysics involves equating the visual effectiveness of brief luminous displays. It may be equally important to equate the effectiveness of brief interruptions, as when two displays are presented sequentially, separated by a variable interstimulus interval (ISI). For example, in a procedure devised by Phillips and Singer [Expl. Brain Res. 19, 493-506 (1974)], the first display consisted of a random pattern of dots and the second display consisted of the same pattern, but with one added dot. Detectability of the added dot was presumed to be determined by interactions of transient neural events produced at the beginning and end of the ISI. Lengthening the ISI was believed to weaken progressively the magnitude of the neural interactions, resulting in poorer performance. But lengthening the ISI also increased its visual effectiveness (darkness). Using ISIs equated in visual effectiveness for durations from 10 to 320 msec, we found that the visual effectiveness of the interval, not its duration, was the prime determinant of performance. This finding requires a reinterpretation of the neural mechanisms being studied in the Phillips and Singer task.

Female↗
PubMed1985

Orienting response and detection of thalamic stimulation: mechanism of perceptual learning in the cat.

Naive cats cannot use thalamic stimulation as a signal to perform a behavioral response when stimulus intensities are too weak to evoke orienting behavior. Responses are quickly learned at higher intensities of stimulation, and with continued training, the cats become able to response to the weaker, previously ineffective stimulus. This increase in sensitivity is not due to changes in tonic arousal and appears to be relatively specific to the stimulated nucleus. The procedures may be useful in exploring the neural mechanisms of perceptual learning.

Animals↗
PubMed1983

Temporal integration following intensification of long-lasting visual displays.

Duration of visible persistence is known to be inversely related to the duration of the inducing stimulus, within a critical interval estimated at between 100 and 150 msec. Stimuli longer than the critical interval yield little or no persistence. Six experiments investigated whether a brief period of intensification at the end of a stimulus longer than the critical interval could restore visible persistence. In the first experiment, a punctate stimulus ceased to give rise to visible persistence at exposure durations longer than the critical interval. The second experiment showed that persistence could be restored to a long display by briefly intensifying the component dots just before the end of the display. The remaining four experiments explored the limits and the distinguishing characteristics of this effect. Two alternative explanations of the results are described and evaluated.

Adaptation, Ocular↗
PubMed1982

Behavioral detection of subcortical stimuli: comparison of somatosensory and "motor" circuits.

Cats were trained to press a lever for food reinforcement in response to stimulation of the ventral lateral (VL) nucleus of the thalamus and the deep cerebellar nuclei. By scaling stimulus intensities relative to the appearance of a minimal amplitude evoked response in pericruciate cortex, it was possible to measure behavioral detection thresholds and correlate behavior with electrocortical activity. With stimulus rates of 25 Hz or greater, VL was the least effective stimulus site for producing detection. At stimulus rates less than 25 Hz, stimulation of the lateral or interpositus nuclei was even less effective in eliciting behavior, but at rates of 25 Hz or more, detection thresholds decreased below those for VL stimulation; cerebellar stimulation produced detection as readily as had stimulation of the ventrobasal complex in other experiments. These findings suggest that the cerebellum may modulate sensory experiences and that some portions of cerebral cortex, the pericruciate and suprasylvian regions, do not appear to be directly involved in mediating sensory detection. It is postulated that the neural detection circuits are more likely to be found in subcortical than in cerebrocortical structures.

Animals↗
PubMed1981

Lesions of the dorsal column nuclei or medial lemniscus of the cat: effect on motor performance.

Motor performance in cats was evaluated by means of a beam-walking task after bilateral lesions were made in dorsal column nuclei (DCN) or medial lemniscus (ML) near its entrance to thalamus. Coordinated motor activity was not significantly impaired by ML lesions or by DCN lesions limited to the main nuclei, but was impaired by larger lesions to DCN that also involved the external cuneate nuclei.

Animals↗
PubMed1980

Detection thresholds to stimulation of ventrobasal complex in cats.

Cats were trained to indicate, by bar pressing for food rewards, their detection of stimulation of the ventrobasal (VB) complex delivered through implanted bipolar electrodes. By varying stimulus intensity it was possible to determine thresholds for detection. Scaling stimulus intensity relative to the appearance of a minimal evoked potential allowed comparisons between animals and also comparisons with results obtained by stimulation of peripheral nerve. Animals could detect VB stimulation, but only at stimulus intensities consistently stronger than those required for minimal appearance of an evoked response in ipsilateral primary somatosensory cortex. Results of VB activation differed from cutaneous nerve effects in that VB detection thresholds were markedly influenced by stimulus frequency. They were lowest at frequencies above 30 Hz and increased greatly at lower frequencies. Discomfort or pain did not seem to result even from relatively high stimulus intensities. The results compare well with observations obtained from stimulation of VB in humans. The appearance of an evoked cortical response is not necessarily correlated with behavior. Under appropriate conditions, behavior can be elicited predictably with minimal electrocortical activity; under other conditions detection may be absent even when large numbers of cortical neurons are activated. We suggest that regions of the cerebral cortex receiving thalamocortical projections from VB may not be essential in the detection process.

Afferent Pathways↗