Biomedical subjects

Jonathan W Peirce

Publications and source records attributed to Jonathan W Peirce.

5 recordsLinked to original sources

PubMed2004

Profound contrast adaptation early in the visual pathway.

Prior exposure to a moving grating of high contrast led to a substantial and persistent reduction in the contrast sensitivity of neurons in the lateral geniculate nucleus (LGN) of macaque. This slow contrast adaptation was potent in all magnocellular (M) cells but essentially absent in parvocellular (P) cells and neurons that received input from S cones. Simultaneous recordings of M cells and the potentials of ganglion cells driving them showed that adaptation originated in ganglion cells. As expected from the spatiotemporal tuning of M cells, adaptation was broadly tuned for spatial frequency and lacked orientation selectivity. Adaptation could be induced by high temporal frequencies to which cortical neurons do not respond, but not by low temporal frequencies that can strongly adapt cortical neurons. Our observations confirm that contrast adaptation occurs at multiple levels in the visual system, and they provide a new way to reveal the function and perceptual significance of the M pathway.

Action Potentials↗

PubMed2004

The impact of suppressive surrounds on chromatic properties of cortical neurons.

Stimulation of the suppressive surround of a cortical neuron affects the responsivity and tuning of the classical receptive field (CRF) on several stimulus dimensions. In V1 and V2 of macaques prepared for acute electrophysiological experiments, we explored the chromatic sensitivity of the surround and its influence on the chromatic tuning of the CRF. We studied receptive fields of single neurons with patches of drifting grating of optimal spatial frequency and orientation and variable size, modulated along achromatic or isoluminant color directions. The responses of most neurons declined as the patch was enlarged beyond the optimal size (surround suppression). In V1 the suppression evoked by isoluminant gratings was less than one-half that evoked by achromatic gratings. Consequently, many cells were most sensitive to achromatic modulation when patches just covered the CRF but were most sensitive to isoluminant modulation when patches were enlarged to cover the suppressive surround. Non-oriented neurons that were strongly chromatically opponent generally lacked suppressive surrounds. In V2 most neurons showed equal surround suppression from isoluminant gratings and achromatic gratings. This makes the relative sensitivity of V2 neurons to achromatic and isoluminant gratings mainly independent of the size of the grating. We also measured the chromatic properties of the CRF in the presence of differently colored surrounds. In neither V1 nor V2 did the surround alter the chromatic tuning of the CRF. Cortical mechanisms sensitive to chromatic contrast seem to provide little input to the suppressive surrounds of V1 neurons but substantial input to those of V2 neurons.

Animals↗

PubMed2002

Functional asymmetry in sheep temporal cortex.

Sheep, like humans, show a bias in favour of the left visual field when discriminating familiar faces. This, in humans, is thought to be caused by a right hemisphere dominance for processing faces involving the temporal cortex. We have directly investigated inter-hemispheric differences in face-processing in sheep by recording the frequencies and response profiles of single cells in the temporal cortex which respond selectively to faces. While there was no evidence for increased frequencies of face-sensitive neurones within the right temporal cortex, or their relative selectivity for individual faces, there was a pronounced response latency difference between the two hemispheres. The cells in the left hemisphere responding selectively to particular faces did so up to 400 ms later than those in the right and with a greater degree of temporal variability between cells. No hemispheric latency differences were found, however, in other cells responding to general visual stimuli or to many faces. The data suggest that, while specialised face-sensitive neural circuits in the right hemisphere may play a key role in the rapid (< 200 ms) identification and discrimination of facial identity, those on the left may be involved more with slower processes associated with integrating the emotional or mnemonic consequences of recognition.

Animals↗

PubMed2002

Binocular integration of partially occluded surfaces.

Normal binocular vision can provide a view of an object partially occluded so that no part of it is seen by both eyes but all of it is seen by one or other eye. We used two-dimensional filtered noise textures to explore the conditions under which the visual system can piece together the monocular fragments of such occluded surfaces. When the fragments seen by left and right eyes are drawn from a continuous texture with strong horizontal correlation, observers see coherent surfaces reliably located in depth. When textures are discontinuous or have weaker horizontal correlation, or the left and right eyes' views represent unnatural depth relationships, no coherent surface is perceived, and binocular rivalry ensues. The discovery of coherent surfaces under our conditions seems to reflect the operation of a high-level integration process, failures of which drive rivalry.

Form Perception↗

PubMedDate not supplied

Residual eye-movements in macaque and their effects on visual responses of neurons.

We recorded continuously, with high precision, the positions of the eyes in anesthetized macaque monkeys prepared for physiological recording. Most recordings were made after the infusion of muscle relaxant to immobilize the eyes; in some cases we also were able to record eye position for periods before the eyes were immobilized. In all monkeys, the eyes moved continuously by as much as 0.5 deg over a 10-min sampling period. The average distance moved was proportional to the square root of the sampling period, as would be expected from a random walk. The movements had three distinct components: slow drifts, and two rhythms driven by the pulse and respiration. The rhythmic movements occurred only under paralysis: they were not discernible in measurements made before the infusion of muscle relaxant. The movements of the eye in the paralyzed animal can have substantial effects on the measured physiological characteristics of neurons. For excursions in the midrange of those we observed, a neuron's sensitivity to a spatial frequency of 10 cycle/deg might be underestimated by as much as a factor of three, depending on the method by which responses were averaged. We show how the effects of eye-movements can be mitigated by appropriate data analysis.

Animals↗