Charles M. Bourassa

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 tablets 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)

Short latency activation of pyramidal tract cells by Group I afferent volleys in the cat

1. The contralateral bulbar pyramids were explored with low impedance micro‐electrodes in cats anaesthetized with chloralose to reveal the effect of Group I afferent volleys (deep radial nerve of the forelimb) on pyramidal tract (Pt) cells.

Detection of stimulus change: the hypothetical roles of visual transient responses

Two random-dot patterns (D1 and D2) were displayed briefly in close temporal succession, separated by an interstimulus interval (ISI). The two patterns were identical except that D2 could contain one more element (addition) or one less element (subtraction) than D1. Observers were required to detect the addition or subtraction using a two-alternative forced-choice method. Three experiments were designed to examine Phillips and Singer’s (1974) suggestion that detection of additions and subtractions can be explained in terms of neural ON and OFF responses. Experiment 1 replicated Phillips and Singer’s finding that performance is progressively impaired as the ISI is increased. In Experiment 2, sudden shifts in luminance between D1 and D2 were combined with several durations of ISI to produce transient responses in a novel manner. Experiment 3 explored the roles of transient neural responses and of configurational changes as determinants of performance. Most predictions based on Phillips and Singer’s model were confirmed by the experimental outcomes. Some discrepancies between theory and data are noted, and directions for further research are suggested.

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.

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 Fechners 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 Fechners paradox than the ganzfelder, but more binocular summation and less Fechners 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.

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.

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.

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.

Comment on reply of wilson and phillips: General agreement

Wilson and Phillips (1987) argue that our data (Bourassa et al., 1985) do not rule out the role of inhibitory interactions between transient On and Off responses in the task we used. On reflection, we tend to agree with them, although we believe the inhibitory interactions described by Wilson and Phillips may not be needed to account for performance on the task. We equated the visual effectiveness, i.e. the darkness, of various interstimulus intervais (ISIS) between the brief appearance of a M-element random-dot display and its reappearance. This was done by adjusting the amount of dimming of the display during the ISI. As expected, to maintain a subjectively uniform level of darkness at all ISIS, the display had to be dimmed less as ISI increased. An experiment was then performed in which the task of the observer was to report whether or not the reappearing display contained an additional dot. We found that, when ISIS were equated, performance was determined by the visual effectiveness of the IS1 and was independent of its duration. This finding led us to conclude that, as performance did not depend on the duration of ISI, interactions between transient On and Off responses were not important in determining the level of performance. Rather, we suggested that the magnitude of the transient On response to the reappearing dots was modulated by the energy level accrued during the period of the IS1 through temporal summation. Wilson and Phillips offer an alternative suggestion concerning the mechanisms by which -.

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.