It is well known that visual cortical neurons respond vigorously to a limited range of stimulus orientations, while their primary afferent inputs, neurons in the lateral geniculate nucleus (LGN) respond well to all orientations. Mechanisms based on intracortical inhibition and/or converging thalamocortical afferents have previously been suggested to underlie the generation of cortical orientation selectivity; however, these models conflict with experimental data. Here, a 1:4 scale model of a 1700m by 200m region of layer IV of cat primary visual cortex (area 17) is presented in order to demonstrate that local intracortical excitation may provide the dominant source of orientation selective input. In agreement with experiment, model cortical cells exhibit sharp orientation selectivity despite receiving strong iso-- orientation inhibition, weak cross--orientation inhibition, no shunting inhibition, and weakly tuned thalamocortical excitation. Sharp tuning is provided by recurrent cortica...

Ganglion cell receptive field centers are small in central retina and larger toward periphery. Accompanying this ex-pansion, the distribution of sensitivity across the centers remain Gaussian, but peak sensitivities decline. To identify circuitry that might explain this physiology, we measured the density of bipolar cell synapses on the dendritic mem-brane of beta (X) and alpha (Y) ganglion cells and the dis-tribution of dendritic membrane across their dendritic fields. Both central and peripheral beta cells receive bipo-lar cell synapses at a density of-2WlOO pm * of dendritic membrane; central and peripheral alpha cells receive-13/ 100 pm*. The distribution of dendritic membrane across the dendritic field is dome-like; therefore, the distribution of bipolar cell synapses is also dome-like. As the dendritic field enlarges, total postsynaptic membrane increases with

the visual response properties of 348 neurons recorded in the lateral geniculate nucleus (LGN) of macaque monkeys aged 1 week to adult. We measured spatial and temporal frequency tuning curves and contrast responses with drifting achromatic sinusoidal gratings. Even in animals as young as 1 week, the main visual response properties of neurons in the magnocellular (M) and parvocellular (P) divisions of the LGN were qualitatively normal, including the spatial organization of receptive fields and the characteristic response properties that differentiate M- and P-cells. At 1 and 4 weeks, spatial and temporal resolution were less than one-half of adult values, whereas contrast gain and peak response rates for optimal stimuli were about two-thirds of adult values. Adult levels were reached by 24 weeks. Analysis of correlations between S-potentials representing retinal inputs and LGN cells suggested that the LGN follows retinal input as faithfully in infants as in adults, implicating retinal development as the main driving force in LGN development. Comparisons with previously published psychophysical data and ideal observer models suggest that the relatively modest changes in LGN responses during maturation impose no significant limits on visual performance. In contrast to previous studies, we conclude that these limits are set by neural development in the visual cortex, not in or peripheral to the LGN. Key words: macaque monkey; lateral geniculate nucleus; development; vision; spatial; temporal

implies that the other bipolar types (b2 and b3) contribute many more quanta to the sustained depolarization (#46 synapse #1 sec #1 ). Type b1 probably contributes large quanta to the transient depolarization. Thus, bipolar cell types b1 and b2/b3 apparently constitute parallel circuits that convey, respectively, high and low frequencies. Key words: quantal rate; ganglion cell; vesicular release; retina; parallel pathways; ribbon synapse Commonly in the CNS, a signal is transmitted forward by multiple pathways operating in parallel. The f unction of such parallelism is rarely apparent, and thus the pathways are often considered merely "redundant." Yet redundancy is an unlikely explanation because there is selective pressure for the brain to use space efficiently (Panico and Sterling, 1995; Hsu et al., 1998). Possibly, parallel pathways might carry different components of the overall signal. Here, I e

ry mechanisms. One integrates inputs linearly across a narrow field (i.e., co-spatial with the ganglion cell's dendritic field), and the other integrates inputs nonlinearly across a wide field (Enroth-Cugell and Robson, 1966; Hochstein and Shapley, 1976a,b; Victor et al., 1977). The response of the linear mechanism can be "nulled": when a visual stimulus (such as a grating) is adjusted so that bright and dark cover equal areas, reversing the contrast evokes no response. The response of the nonlinear mechanism cannot be nulled: the cell fires at each contrast reversal, i.e., at twice the stimulus cycle. The nonlinear mechanism resolves a much finer grating (higher spatial frequency) than the linear mechanism, suggesting that the nonlinear mechanism is composed of multiple spatial "subunits" (Hochstein and Shapley, 1976b; Derrington et al., 1979). The linear mechanism has been attributed to bipolar cells (Hochstein and Shapley, 1976b; Victor et al., 1977; Victor and Shapley, 1979), beca

Introduction The quality of the visual signal transmitted to the brain is important for perception because it sets the minimum detectable stimulus. As a daylight visual signal is processed by the retina, each layer adds noise (Ashmore and Copenhagen, 1983; Freed et al., 2003) so that the signal quality of a ganglion cell is limited by retinal noise sources (Schellart and Spekreijse, 1973; Reich et al., 1977; Levine and Zimmerman, 1991; Troy and Robson, 1992; Croner et al., 1993; Freed, 2000), implying information loss (Geisler, 1989). The loss is thought to originate partly in selective processing of the signal and partly from noise sources such as stochastic vesicle release and channel gating (Barrett and Stevens, 1972; Schneidman et al., 1998; White et al., 2000; van Rossum et al., 2003). To understand how efficiently information is transferred and how neural mechanisms preserve signal quality, one could measure information loss at each retinal stage. One way is to compare the contr