We describe a new, computationally simple method for analyzing the dynamics of neuronal spike trains driven by external stimuli. The goal of our method is to test the predictions of simple spike-generating models against extracellularly recorded neuronal responses. Through a new statistic called the power ratio, we distinguish between two broad classes of responses: (1) responses that can be completely characterized by a variable firing rate, (for example, modulated Poisson and gamma spike trains); and (2) responses for which firing rate variations alone are not sufficient to characterize response dynamics (for example, leaky integrate-and-fire spike trains as well as Poisson spike trains with long absolute refractory periods). We show that the responses of many visual neurons in the cat retinal ganglion, cat lateral geniculate nucleus, and macaque primary visual cortex fall into the second class, which

We demonstrate that the information contained in the spike occurrence times of a population of neurons can be broken up into a series of terms, each of which reflect something about potential coding mechanisms. This is possible in the coding r'egime in which few spikes are emitted in the relevant time window. This approach allows us to study the additional information contributed by spike timing beyond that present in the spike counts; to examine the contributions to the whole information of different statistical properties of spike trains, such as firing rates and correlation functions; and forms the basis for a new quantitative procedure for the analysis of simultaneous multiple neuron recordings. It also provides theoretical constraints upon neural coding strategies. We find a transition between two coding r'egimes, depending upon the size of the relevant observation timescale. For time windows shorter than the timescale of the stimulus-induced response fluctuations, t...

words: interval; short-term plasticity; paired-pulse facilitation; paired-pulse depression; timing; temporal processing Our perception of the world is based on the spatiotemporal patterns of neuronal activity produced at sensory layers. By decoding these patterns the brain determines what we see and hear. It is useful to distinguish between the spatial and temporal content of stimuli because f undamentally different mechanisms may underlie each form of processing. Spatial information refers to stimuli defined by the location of active sensory afferents. For instance, vertical and horizontal bars of light activate different retinal ganglion cells arranged in specific spatial patterns. Similarly, 1 and 4 kHz tones activate spatially distinct populations of cochlear hair cells. In both cases there is a place code at the earliest sensory stages. In its simplest form, the generation of neurons that respond selectively to spatial stimuli is a wiring problem: a neuron that responds t

Edge-like and line-like features result from spatial phase congruence, the local phase agreement between harmonic components of a spatial waveform. Psychophysical observations and models of early visual processing suggest that human visual feature detectors are specialized for edge-like and linelike phase congruence. To test whether primary visual cortex (V1) neurons account for such specificity, we made tetrode recordings in anesthetized macaque monkeys. Stimuli were drifting equal-energy compound gratings composed of four sinusoidal components. Eight congruence phases (onedimensional features) were tested, including line-like and edgelike waveforms. Many of the 137 single V1 neurons (recorded at 45 sites) could reliably signal phase congruence by any of several response measures. Across neurons, the preferred spatial feature had only a modest bias for line-like waveforms.

In the primary visual cortex (V1) the contrast response function of many neurons saturates at high contrast and adapts depending on the visual stimulus. We propose that both effects---contrast saturation and adaptation---can be explained by a fast and a slow component in the synaptic dynamics. In our model the saturation is an effect of fast synaptic depression with a recovery time constant of about 200 ms. Fast synaptic depression leads to a contrast response function with a high gain for only a limited range of contrast values. Furthermore, we propose that slow adaptation of the transmitter release probability at the geniculocortical synapses is the underlying neural mechanism that accounts for contrast adaptation on a time scale of about 7 sec. For the functional role of contrast adaptation we make the hypothesis that it serves to achieve the best visual cortical representation of the geniculate input. This representation should maximize the mutual information between the cortical a...

y to describe the main, usual form (or forms) of communication. We should take the approach of the moderately bright investigator, and leave the discovery of exceptional facts for later on. Further, we should try to quantify how much is communicated in each situation, because only a quantitative comparison allows to assess different codes, especially if they share part of the content of what is being communicated. Information theory [1] has been developed precisely to quantify communication, and is therefore quintessential to an appraisal of neural codes. Applying information theory to neural activity (rather than to the synthetic communication systems for which it was developed) is however riddled with practical problems and subtleties, which must be clarified before reporting experimental results. In this chapter, we do not consider other means of neuronal communication than the emission of action potentials, or spikes, and regard them as selfsimilar all-or-none even

We studied the spatial organization of receptive fields and the responses to gratings of neurons in parafoveal V1 of alert monkeys. Activating regions (ARs) of 228 cells were mapped with increment and decrement bars while compensating for fixational eye movements. For cells with 2 or more ARs, the overlap between ARs responsive to increments (INC) and ARs responsive to decrements (DEC) was characterized by a quantitative overlap index (OI; Schiller et al. 1976). The distribution of overlap indices was bimodal. The larger group (78% of cells) was comprised of complex cells with strongly overlapping ARs (OI >=0.5). The smaller group (14%) was comprised of simple cells with minimal spatial overlap of ARs (OI 1, the traditional criterion for identifying simple cells. However, unlike simple cells, even those complex cells with high RM could exhibit diverse nonlinear responses when the spatial frequency or window size was changed. Furthermore, the responses of complex cells to counterphase gratings were predominantly nonlinear even harmonics. These results show that RM is not a robust test of linearity. Our results indicate that complex cells are the most frequently encountered neurons in primate V1, and their behavior needs to receive more emphasis in models of visual function.

JN-00829-2003.R1 2 We measured the information available for orientation discrimination from metric distances for 24 cells in area 17 of cats that were paralyzed and anesthetized with propofol and N2O. The metric distance information confirms fundamental coding differences for discrimination between fine (<10º) and coarse (>10º) orientation differences. The information for discriminating larger orientation differences is contained mainly in the firing rate, with minor enhancements from coarse (30-70 ms) temporal structure in the firing rate. Both precise spike timing (9.2 ms) and intervals (6.8 ms) sustained over the stimulus presentation provide information for fine discrimination of orientation, where almost no reliable information is provided by the spike count. We compare and confirm the results (using the same dataset) to vector distances based on classification theory (Samonds et al. 2003a). The results support a dynamic spiking mechanism where coordinated activity could provide fast and reliable information about detailed angle and/or direction information in the region of the preferred orientation.

Response reliability is complementary to more conventional measurements of response amplitudes, and can reveal phenomena that response amplitudes do not. Here we review studies that measured reliability of cortical activity within or between human subjects in response to naturalistic stimulation (e.g. free viewing of movies). Despite the seemingly uncontrolled nature of the task, some of these complex stimuli evoke highly reliable, selective and time-locked activity in many brain areas, including some regions that show little response modulation in most conventional experimental protocols. This activity provides an opportunity to address novel questions concerning natural vision, temporal scale of processing, memory and the neural basis of inter-group differences.

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