Abstract
Purpose: :
The frontal eye field (FEF) is a region of prefrontal cortex that plays an important role in vision and eye movements. Like other visual areas, single neurons in FEF are responsive to stimuli in specific regions of visual space, known as the receptive field (RF). Conventional RF mapping protocols are limited in their ability to capture the dynamic nature of neuronal RFs. RFs are generally measured outside the context of the central experimental manipulation, sometimes in greatly simplified paradigms. Such surrogate RF measures depend on the assumption that RFs remain stable across conditions and attentional states. We set out to devise a more direct measure of RF dynamics in FEF.
Methods: :
We used a slower frame rate version of "spike-triggered averaging" (STA), a technique established to measure RFs in primary visual cortex. Stimuli consisted of small white dots on a black background, presented pseudorandomly at variable locations on the screen while animal subjects maintained fixation. We extracellularly recorded the visual activity of single FEF neurons in response to STA stimuli. As a control, we also measured RFs using the conventional method of presenting a single stimulus at regularly spaced locations of fixed directions and amplitudes over several trials.
Results: :
We recorded from over 25 FEF neurons and measured RFs using rapidly presented stimuli. STA and conventional measures of RF centers differed, on average, by only 6.4 degrees. RF sizes were also similar across methodologies. For a subset of neurons (n = 9), we measured temporal sensitivity by using both "fast" and "slow" (67 and 150 ms frame durations, respectively) versions of the STA movies. The majority of FEF neurons were robust to such temporal changes. Finally, when conventional methods failed to find a RF, STA-based methods generally also failed to find a RF.
Conclusions: :
We devised a more direct measure of RF dynamics in FEF. STA methods can be used to map the spatial and temporal properties of FEF RFs in a relatively unbiased manner and can likely be extended to include saccadic RFs. Our findings validate the STA method of RF mapping in prefrontal cortex and will assist in uncovering the neuronal circuitry supporting visual perception.
Keywords: eye movements • visual cortex • vision and action