Standardized F1 – A consistent measure of strength of modulation of visual responses to sine-wave drifting gratings

Abstract

The magnitude of spike-responses of neurons in the mammalian visual system to sine-wave luminance-contrast-modulated drifting gratings is modulated by the temporal frequency of the stimulation. However, there are serious problems with consistency and reliability of the traditionally used methods of assessment of strength of such modulation. Here we propose an intuitive and simple tool for assessment of the strength of modulations in the form of standardized F1 index, zF1. We define zF1 as the ratio of the difference between the F1 (component of amplitude spectrum of the spike-response at temporal frequency of stimulation) and the mean value of spectrum amplitudes to standard deviation along all frequencies in the spectrum. In order to assess the validity of this measure, we have: (1) examined behavior of zF1 using spike-responses to optimized drifting gratings of single neurons recorded from four 'visual' structures (area V1 of primary visual cortex, superior colliculus, suprageniculate nucleus and caudate nucleus) in the brain of commonly used visual mammal - domestic cat; (2) compared the behavior of zF1 with that of classical statistics commonly employed in the analysis of steady-state responses; (3) tested the zF1 index on simulated spike-trains generated with threshold-linear model. Our analyses indicate that zF1 is resistant to distortions due to the low spike count in responses and therefore can be particularly useful in the case of recordings from neurons with low firing rates and/or low net mean responses. While most V1 and a half of caudate neurons exhibit high zF1 indices, the majorities of collicular and suprageniculate neurons exhibit low zF1 indices. We conclude that despite the general shortcomings of measuring strength of modulation inherent in the linear system approach, zF1 can serve as a sensitive and easy to interpret tool for detection of modulation and assessment of its strength in responses of visual neurons.

Spectral Characteristics of Phase Sensitivity and Discharge Rate of Neurons in the Ascending Tectofugal Visual System

“…The discharge-rate spectral RFs provide information about the spatiotemporal properties of the neurons [12], [15], [49]–[53], while F1 spectral RFs bring information about their phase sensitivity [53]. Since F1 depends on the magnitude of the response, we have used standardized F1 values [40] to plot spectral RFs.…”

“…Earlier studies revealed stimulus-dependent modulations of neuronal activity in the SC [15], [40], [62], LP-Pul complex [35], Sg [16], [40], as well as from the lateral most part of the AES cortex (insular cortex) [37], and the CN [36], [40]. However, to our knowledge, there are no studies that have examined modulatory spectral RFs or compared spatial and temporal frequencies in response to an optimal stimulus in terms of discharge rate versus those maximizing modulations in the extrageniculate visual system.…”

“…We have previously shown that zF1 is a suitable tool for the detection and quantitative assessment of neuronal modulation at the stimulus TF even for neurons with low firing rates and/or a low net magnitude of responses [40]. The proportion of cells exhibiting oscillatory modulations at the TF of the stimulus varied between tectofugal structures.…”

“…Such data standardization is widely used and is commonly referred to as the ‘z-score’. Previously we showed that zF1 is a reliable measure of the oscillatory modulations in neuronal responses and can be successfully applied to spike train analyses of single visual neurons recorded during stimulation with drifting gratings [40].…”

“…SCs [15], LP-Pul [35] or the Sg, [16], as well as their subcortical and cortical projection targets: the dorsolateral segment of caudate nucleus of the neostriatum (CN) [36], insular cortex [37] and the lateral suprasylvian cortex [18], [19], [38], and area 21b [39]. The number of neurons exhibiting an oscillatory modulation of spike rate and the degree of modulation were found to be much lower in extrageniculate pathway structures compared with area 17 [18], [19], [38]–[40]. This may indicate a much higher content of nonlinearities in the responses of the extrageniculate pathway.…”