Biomedical subjects

Timothy J Gawne

Publications and source records attributed to Timothy J Gawne.

5 recordsLinked to original sources

PubMed2003

The responses of visual cortical neurons encode differences across saccades.

Primate vision consists mostly of periods of stable fixation separated by rapid saccadic eye movements. Each saccade brings a new scene onto the retina, and each new scene results in a burst of activity in the neurons of visual cortex. It might be expected that the activity of these neurons should only represent what is on the retina now, much as a video camera hooked up to a television only displays what the camera is currently pointed at. However, we show here that this is not the case. Recording from 25 primary visual cortical neurons in an awake primate demonstrated that the responses to the saccade-induced presentation of a stimulus within a neuron's receptive field (RF) are typically suppressed by the presence of a stimulus in the RF before the saccade. Flashing stimuli on with the eyes stationary showed, on average, suppressive effects of similar magnitude, suggesting that the mechanism is simple adaptation. However, while the mechanism may be simple, the implications for the operation of the visual system are not. The activity of visual cortical neurons does not represent just the current retinal image, but also the differences between the current retinal image and the previous one. These results suggest that the current approach of studying the visual system, which concentrates on determining the relationship between a single stimulus and a single response, may have to be modified to take into account the timing of retinal image changes that occurs in normal vision.

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PubMed2002

Responses of primate visual cortical neurons to stimuli presented by flash, saccade, blink, and external darkening.

Our visual experience constitutes an unending chain of transient events, including those caused by saccadic eye movements, by blinks, and by localized or global changes in the external world. The categorical perception of objects is maintained across different classes of transient events, suggesting that the neural circuitry underlying visual perception responds to different transient events in a similar manner. However, different sorts of transients do have different perceptual impacts: for example, the sudden changes in a scene due to a saccade or a blink do not disturb our perceptual continuity of a visual scene as much as an external change does. We recorded the responses of 103 single visual cortical neurons in two rhesus monkeys (V1: n = 38, V2: n = 19, V3V/VP: n = 30, V4V: n = 16) to the onset and offset of a visual stimulus that was elicited by four different conditions: 1) stimulus flashed on and off while the eyes remain fixed; 2) stimulus turned on and off along with the entire scene (external darkening); 3) stimulus constant, onset and offset induced by rapid saccadic eye movements; and 4) offset induced by an eyeblink. For most neurons the onset and offset of a visual stimulus elicited qualitatively similar responses regardless of condition. We found no systematic effect of different conditions across the neuronal population. Previously we have shown that when the visual scene is occluded by a blink V1 neuronal firing declines in a similar manner as when the external scene is darkened and the eyes left open. Here we show that this is also the case in V2, V3V/VP, and V4V. However, for a substantial minority of neurons, the response varied strongly as a function of the transient event. This overall pattern was the same in all four cortical areas studied here. We hypothesize that most neurons in visual cortex constitute a passive "filter bank", analyzing the scene for specific details regardless of condition. However, there are neurons that respond in a qualitatively different manner depending on how a stimulus is presented, and we hypothesize that these signals may be important for determining the perceptual salience of a visual event.

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PubMed2002

Responses of primate visual cortical V4 neurons to simultaneously presented stimuli.

We report here results from 45 primate V4 visual cortical neurons to the preattentive presentations of seven different patterns located in two separate areas of the same receptive field and to combinations of the patterns in the two locations. For many neurons, we could not determine any clear relationship for the responses to two simultaneous stimuli. However, for a substantial fraction of the neurons we found that the firing rate was well modeled as the maximum firing rate of each stimulus presented separately. It has previously been proposed that taking the maximum of the inputs ("MAX" operator) could be a useful operation for neurons in visual cortex, although there has until now been little direct physiological evidence for this hypothesis. Our results here provide direct support for the hypothesis that the MAX operator plays a significant (although certainly not exclusive) role in generating the receptive field properties of visual cortical neurons.

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PubMedDate not supplied

Video-rate and continuous visual stimuli do not produce equivalent response timings in visual cortical neurons.

Video cathode ray tube (CRT) technology has proven to be extremely valuable for performing research in the visual system. However, the image on a CRT monitor is not constant, but consists of a series of brief pulses. This has implications for any study that explores the responses of neurons in the visual system on short time scales. In particular, there is no unambiguous time point at which a visual stimulus presented via CRT may be said to have ended. Recordings from single units in visual cortical area V1 of an awake primate demonstrate that, when studying changes in response timing on the order of 10 ms or less, stimuli delivered at video frame rates do not duplicate the effects seen with stimuli that have continuous functions of luminance versus time. Additionally, there does not seem to be any clear method of comparing the results obtained with video-rate stimuli with results obtained with continuous-time stimuli that holds for all conditions. These effects are especially critical when exploring the time course of the neuronal responses to the ending of a visual stimulus (off-response). Our findings cast doubt upon the recently reported result that off-responses have consistently shorter latencies than on-responses.

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PubMedDate not supplied

Retinal ganglion cell coding in simulated active vision.

The image on the retina is almost never static. Eye, head, and body movements, and externally generated motion create rapid and continual changes in the retinal image ("active vision"). Virtually all vision in animals such as primates, which make saccades as often as 3-4 times/s, is based on information that must be derived from the first few hundred milliseconds after sudden, global changes in the retinal image. These changes may be accompanied by large changes in area mean luminance, as well as higher order image contrast statistics. This study investigated how retinal ganglion cell responses, whose response properties have been typically studied and defined in a stable stimulus regime, are affected by sudden changes in mean luminance that are characteristic of active vision. Specifically, the steady-state responses of retinal ganglion cells to static or moving square-wave grating stimuli were recorded in an isolated, superfused rabbit eyecup preparation and compared to responses after saccade-like changes in luminance. The manner of coding after luminance changes was different for different ganglion cell classes; both suppression and enhancement of responses to patterns following luminance changes were found. Brisk-transient Off cells unambiguously signaled the darkening of the overall image, but were also modulated by the subsequently appearing grating stimulus. Several types of On-center cell behavior were observed, ranging from strong suppression of the subsequent response by luminance changes, to strong enhancement. Overall, most ganglion cells distinguished static patterns after a luminance change via differences in their spike discharges nearly as well as before, although there were clear asymmetries between the On and Off pathways. Changes in mean luminance in some ganglion cells, such as On-Off directionally selective ganglion cells, could create large phase shifts in the response to patterned, moving stimuli, although these stimuli were still detected immediately after luminance changes. The results of this study show that the image dynamics of active vision may be a fundamental challenge for the visual system because of strong effects on retinal ganglion cell function. However, rapid extraction of unambiguous information after luminance changes appears to be encoded in differences in the spike discharges in different retinal ganglion cell classes. Asymmetries among ganglion cell classes in sensitivity to luminance changes may provide a basis by which some provide the "context" for interpreting the firing of others.

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