On interpreting responses to low contrast stimuli in terms of magnocellular activity – A few remarks

Skottun & Skoyles [Skottun and Skoyles (2009). On interpreting responses to low contrast stimuli in terms of magnocellular activity – a few remarks. Vision Research] have written a comment on our recent paper (Lalor & Foxe, 2009). Specifically they raise the following three issues: (1) spatial resolution and temporal tuning are similar in magnoand parvocellular neurons. (2) Results from lesion studies make reliance upon low contrast to isolate magnocellular activity problematic. (3) Other neurons exist which have contrast-response functions similar to those of magnoand parvocellular cells and, ‘‘[t]hus, it seems difficult to differentiate, on the basis of contrast gain and saturation, contributions that are uniquely magnoand parvocellular from cortical activity”. We respond that issues (1) and (2) have not only been called into question several times previously, but that they are irrelevant to the main manipulation carried out in our study. Despite this, we would like to take the opportunity to demonstrate that these arguments are based on a rather selective interpretation of the literature. We feel that we have not contradicted issue (3) in our original article and, furthermore, that the points made are insubstantial. In our paper (Lalor & Foxe, 2009), we presented Visual Evoked Spread Spectrum Analysis (VESPA) responses to four different stimulus types. In each case, the basic stimulus consisted of a checkerboard pattern subtending visual angles of 5.25 vertically and horizontally, with equal numbers of light and dark checks. The refresh rate of the monitor was set to 60 Hz and on every refresh the contrast of the checkerboard pattern was modulated by a non-binary stochastic signal which had its power distributed uniformly between 0 and 30 Hz. Responses were then obtained by linear least squares estimation on the assumption that the output EEG represented a convolution of the stochastically modulated contrast signal with an unknown impulse response, namely, the VESPA (Lalor, Pearlmutter, & Foxe, 2009; Lalor, Pearlmutter, Reilly, McDarby, & Foxe, 2006). For three of the four stimulus types, each individual check subtended a visual angle of 0.65 and the only manipulation that was carried out was related to the range of contrasts over which the checkerboard was modulated. In the FULLRANGE case, the checkerboard was modulated between 0% and 100% using a Gaussian random process with a zero-point corresponding to the mid-point of the range (i.e., 50% contrast) and with a scaling that allowed ±3 standard deviations within the range. In the HIGH-CONTRAST case, the checkerboard was again modulated by a Gaussian random process, but the range was restricted to between 32% and 100%. Again the mid-point of this range corresponded to the zero-point of our mapping with ±3 standard deviations allowed within the range. The LOW-CONTRAST stimu-