To extract important information from the environment on a useful timescale, the visual system must be able to adapt rapidly to constantly changing scenes. This requires dynamic control of visual resolution, possibly at the level of the responses of single neurons. Individual cells in the visual cortex respond to light stimuli on particular locations (receptive fields) on the retina, and the structure of these receptive fields can change in different contexts. Here we show experimentally that the shape of receptive fields in the primary visual cortex of anaesthetized cats undergoes significant modifications, which are correlated with the general state of the brain as assessed by electroencephalography: receptive fields are wider during synchronized states and smaller during non-synchronized states. We also show that cortical receptive fields shrink over time when stimulated with flashing light spots. Finally, by using a network model we account for the changing size of the cortical receptive fields by dynamically rescaling the levels of excitation and inhibition in the visual thalamus and cortex. The observed dynamic changes in the sizes of the cortical receptive field could be a reflection of a process that adapts the spatial resolution within the primary visual pathway to different states of excitability.

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The mean tonic activity of five X-and five Y-cells is plotted over time after onset of QUIN application with 10-20 nA. The curves were slightly smoothed by calculating sliding means from three successive responses. The responses were normalized to the mean of the three first response levels, which were set to 0 on the graph. Continuous lines show Y-cell activity, dashed lines that for the X-cells. The grey lines show overall means for X-and Y-cells. Regression lines have also been added.

Adding noise to a weak signal can paradoxically improve signal detection, a process called 'stochastic resonance' (SR). In the visual system, noise might be introduced by the image jitter resulting from high-frequency eye movements, like eye microtremor and microsaccades. To test whether this kind of noise might be beneficial or detrimental for cortical signal detection, we performed single-unit recordings from area 17 of anaesthetized cats while jittering the visual stimulus in a frequency and amplitude range resembling the possible range of eye movements. We used weak, sub- and peri-threshold visual stimuli, on top of which we superimposed noise with variable jitter amplitude. In accordance with the typical SR effect, we found that small noise levels actually increased the signal-to-noise ratio (SNR) of previously weak cortical visual responses, while originally strong responses were little affected or even reduced. Above a certain noise level, the SNR dropped a little, but not as a result of increased background activity - as would be proposed by SR theory - but because of a lowered response to signal and noise. Therefore, it seems that the ascending visual pathway optimally utilizes signal detection improvement by a SR-like process, while at the same time preventing spurious noise-induced activity and keeping the SNR sufficiently high.

We investigated how changes in the temporal firing rate of thalamocortical activity affect the spatiotemporal structure of receptive field (RF) subunits in cat primary visual cortex. Spike activity of 67 neurons (48 simple, 19 complex cells) was extracellulary recorded from area 17/18 of anesthetized and paralyzed cats. A total of 107 subfields (on/off) were mapped by applying a reverse correlation technique to the activity elicited by bright and dark rectangles flashed for 300 ms in a 20x10 grid. We found that the width of the (suprathreshold) discharge fields shrank on average by 22% during this 300-ms-long stimulus presentation time. Fifty-eight subfields (54%) shrank by more than 20% of peak width and only ten (less than 10%) showed a slight increase over time. The main size reduction took place 40-60 ms after response onset, which corresponded to the transition from transient peak firing to tonic visual activity in thalamocortical relay cells (TC). The experimentally obtained RFs were then fitted with the aid of a neural field model of the primary visual pathway. Assuming a Gaussian-shaped spatial sensitivity profile across the RF subfield width, the model allowed us to estimate the subthreshold RF (depolarization field, D-field) from the minimal discharge field (MDF). The model allowed us to test to what degree the temporal dynamics of thalamocortical activity contributes to the spatiotemporal changes of cortical RFs. To this end, we performed the fitting procedure either with a pure feedforward model or with a field model that also included intracortical feedback. Spatial and temporal parameters obtained from fits of the experimental RFs matched closely to those achieved by simulating a pure feedforward system with the field model but were not compatible with additional intracortical feedback. Thus, our results show that dot stimulation, which optimally excites thalamocortical cells, leads to a shrinkage with respect to the size of the RF subfield at the first transient response of visual cortical RFs which seems mainly due to a change in the thalamic firing pattern. In these experiments little or no influence from intracortical sources was observed, which, however, may play a role when using more complex visual stimuli.