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.

Adaptation to leftward-shifting prisms reduces the global processing bias of healthy individuals.

When healthy individuals are presented with peripheral figures in which small letters are arranged to form a large letter, they are faster to identify the global- than the local-level information, and have difficulty ignoring global information when identifying the local level. The global reaction time (RT) advantage and global interference effect imply preferential processing of global-level information in the normal brain. This contrasts with the local processing bias demonstrated following lesions to the right temporo-parietal junction (TPJ), such as those that lead to hemispatial neglect (neglect). Recent research from our lab demonstrated that visuo-motor adaptation to rightward-shifting prisms, which ameliorates many leftward performance deficits of neglect patients, improved the local processing bias of patients with right TPJ lesions (Bultitude, Rafal, & List, 2009). Here we demonstrate that adaptation to leftward-shifting prisms, which can induce neglect-like performance in neurologically healthy individuals, also reduces the normal global processing bias. Forty-eight healthy participants were asked to identify the global or local forms of hierarchical figures before and after adaptation to leftward- or rightward-shifting prisms. Prior to prism adaptation, both groups had greater difficulty ignoring irrelevant global information when identifying the local level (global interference) compared to their ability to ignore irrelevant local-level information when identifying the global level (local interference). Participants who adapted to leftward-shifting prisms showed a significant reduction in global interference, but there was no change in the performance of the rightward-shifting Prism Group. These results show, for the first time, that in addition to previously demonstrated effects on lateralised attention, prism adaptation can influence non-lateralised spatial attention in healthy individuals.

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.