The primary visual cortex (V1) is the ¢rst cortical area to receive visual input, and inferior temporal (IT) areas are among the last along the ventral visual pathway. We recorded, in area V1 of anaesthetized cats and area ITof awake macaque monkeys, responses of neurons to videos of natural scenes. Responses were analysed to test various hypotheses concerning the nature of neural coding in these two regions. A variety of spike-train statistics were measured including spike-count distributions, interspike interval distributions, coe¤cients of variation, power spectra, Fano factors and di¡erent sparseness measures. All statistics showed non-Poisson characteristics and several revealed self-similarity of the spike trains. Spike-count distributions were approximately exponential in both visual areas for eight di¡erent videos and for counting windows ranging from 50 ms to 5 seconds. The results suggest that the neurons maximize their information carrying capacity while maintaining a ¢xed long-term-average ¢ring rate, or equivalently, minimize their average ¢ring rate for a ¢xed information carrying capacity. I N T RO DUC T IONIt has been suggested that visual representations are optimized to transmit the maximum information about the images encountered in everyday life (Uttley 1973;Linsker 1987;Barlow 1989). This simple assumption has proven su¤cient to account for the characteristics of large monopolar cells in the £y (Srinivasan et al. 1982;Van Hateren 1992;Laughlin 1981), the temporal characteristics of retinal ganglion cells (Dong & Atick 1995), human spatial frequency thresholds (Atick & Redlich 1992;Van Hateren 1993), and the psychophysics of orientation perception for short presentation times (Baddeley & Hancock 1991).Maximization of information is a powerful theoretical principle that leads to testable predictions about the ¢ring patterns of neurons. However, to generate speci¢c predictions we must make some assumptions about the nature of the neural code and the type of constraint that limits its information carrying capacity. To apply information maximization to neuronal spike trains, we must identify which of their characteristics carry information. In our analysis, we will consider two possibilities: that ¢ring rates, or more precisely, spike counts over discrete intervals of time, are the information carrying elements; or that interspike intervals play this role. Without any constraints on the rate or precision of neuronal spiking, the information carrying capacity of a spike train is in¢nite. Thus, constraints play a crucial role in any information maximization procedure. We will consider three possibilities, constraints on the maximum ¢ring rate, the average ¢ring rate, or a quantity known as the sparseness of the ¢ring-rate distribution. Identifying the nature of the constraint that limits information carrying capacity has important implications for the biophysical mechanisms that underlie neural coding.Assuming the ¢ring rates carry information, Laughlin (1981) proposed a constraint on the maximum ¢...

The effects of stimuli falling outside the 'classical receptive field' and their influence on the orientation selectivity of cells in the cat primary visual cortex are still matters of debate. Here we examine the variety of effects of such peripheral stimuli on responses to stimuli limited to the receptive field. We first determined the extent of the classical receptive field by increasing the diameter of a circular patch of drifting grating until the response saturated or reached a maximum, and by decreasing the diameter of a circular mask in the middle of an extended grating, centred on the receptive field, until the cell just began to respond. These two estimates always agreed closely. We then presented an optimum grating of medium-to-high contrast filling the classical receptive field while stimulating the surround with a drifting grating that had the same parameters as the central stimulus but was varied in orientation. For all but five neurons (of 37 tested), surround stimulation produced clear suppression over some range of orientations, while none showed explicit facilitation under these conditions. For 11 cells (34% of those showing suppression), the magnitude of suppression did not vary consistently with the orientation of the surround stimulus. In the majority of cells, suppression was weakest for a surround grating oriented orthogonal to the cell's optimum. Nine of these cells (28%) exhibited maximum inhibition at the optimum orientation for the receptive field itself, but for 12 cells (38%) there was apparent 'release' from inhibition for surround gratings at or near the cell's optimum orientation and direction, leaving inhibition either maximal at angles flanking the optimum (9 cells) or broadly distributed over the rest of the orientation range (3 cells). This implies the existence of a subliminal facilitatory mechanism, tightly tuned at or near the cell's optimum orientation, extending outside the classical receptive field. For just two cells of 13 tested the preferred orientation for a central grating was clearly shifted towards the orientation of a surrounding grating tilted away from the cell's optimum. The contrast gain for central stimulation at the optimal orientation was measured with and without a surround pattern. For nine of 25 cells tested, surround stimulation at the cell's optimum orientation facilitated the response to a central grating of low contrast (< or =0.1) but inhibited that to a higher-contrast central stimulus: the contrast-response gain is reduced but the threshold contrast is actually decreased by surround stimulation. Hence the receptive field is effectively larger for low-contrast than for high-contrast stimuli. Inhibition from the periphery is usually greatest at or around the cell's optimum, while suppression within the receptive field has been shown to be largely non-selective for orientation. Inhibition by orientations flanking the optimum could serve to sharpen orientation selectivity in the presence of contextual stimuli and to enhance orientational contrast; and ...

Experience is known to affect the development of ocular dominance maps in visual cortex, but it has remained controversial whether orientation preference maps are similarly affected by limiting visual experience to a single orientation early in life. Here we used optical imaging based on intrinsic signals to show that the visual cortex of kittens reared in a striped environment responded to all orientations, but devoted up to twice as much surface area to the experienced orientation as the orthogonal one. This effect is due to an instructive role of visual experience whereby some neurons shift their orientation preferences toward the experienced orientation. Thus, although cortical orientation maps are remarkably rigid in the sense that orientations that have never been seen by the animal occupy a large portion of the cortical territory, visual experience can nevertheless alter neuronal responses to oriented contours.

We summarize here the results presented and subsequent discussion from the meeting on Integrating Hebbian and Homeostatic Plasticity at the Royal Society in April 2016. We first outline the major themes and results presented at the meeting. We next provide a synopsis of the outstanding questions that emerged from the discussion at the end of the meeting and finally suggest potential directions of research that we believe are most promising to develop an understanding of how these two forms of plasticity interact to facilitate functional changes in the brain.This article is part of the themed issue 'Integrating Hebbian and homeostatic plasticity'.