Third, the iso-response

measurements can assess nonlinearities of stimulus integration by retinal ganglion cells independent of the cell’s intrinsic nonlinear processing. This cell-intrinsic nonlinearity implicates, for example, that it is typically not possible to check for linear summation of inputs by comparing the response for multiple simultaneous stimulus components to the sum of responses for the individual components. Such a measurement would require an accurate model of cell-intrinsic signal processing in order to tease apart the different nonlinearities that ultimately affect the response. Fourth, focusing on a fixed response level Linsitinib naturally keeps the neuron close to a constant adaptation level and thus minimizes confounding adaptation effects, as might result from sporadically driving the neuron at particularly high firing rates. And fifth, iso-response stimuli seem a natural way for investigating the dimensional reduction that results when neurons integrate several inputs and map these inputs onto a low-dimensional response, such as the neuron’s spike count. A fundamental consequence is that different input patterns will be mapped click here onto the same output. This contributes to establishing invariances, which represent a hallmark of neural computation (Riesenhuber and Poggio, 2000) and underlie complex recognition and decision tasks. It thus appears

appropriate to assess computation at the single-neuron level also by identifying which stimuli are classified as equal. Indeed, measuring iso-response stimuli can provide a new perspective

on nonlinear signal integration not apparent in other, standard approaches. For example, a simple model simulation shows that homogeneity detectors look just like typical Y-type cells for contrast-reversing gratings (Figure S1 available online), the classical stimulus to test for nonlinear spatial integration. A caveat of the closed-loop experiments is that they rely on accurate online detection nearly of the incoming signals, here the ganglion cell spikes. Systematic errors in spike detection could, in principle, lead the search for the predefined response astray. We avoided such pitfalls by selecting only ganglion cells whose spikes were sufficiently large for simple and unambiguous detection through threshold crossing. In addition, we verified the accuracy of the online spike detection by additional in-depth offline analysis of the spike waveforms. The selection of large and reliable spikes, however, may add to a potential recording bias (Olshausen and Field, 2005); ganglion cell types with small cell bodies, for example, might not always create spikes with sufficient size in the extracellular recordings (Towe and Harding, 1970 and Olshausen and Field, 2005) to pass our criterion of reliable spike detection and may therefore be underrepresented in our analysis.