Research Highlight:

A Searching Heirarchy

Original Research Article:
JY Cohen, RP Heitz, JD Schall and GF Woodman (2009). On the origin of event-related potentials indexing covert attentional selection during visual search. J Neurophysiol. 102 (4): 2375-2386.

Electroencephalography (EEG) is one of the most popular functional imaging techniques in psychology, neurology, and neuroscience. Compared to methods such as magnetic resonance imaging (MRI) and positron emission tomography (PET), EEG has unparalleled advantages in temporal resolution and cost. The blaring weakness of EEG is the inverse problem which leads to a deficiency in spatial resolution. Cohen and colleagues tackle this impasse in their recent Journal of Neurophysiology report. The researchers combined measurements of intracranial single-neuron spikes, local field potentials (LFPs), and event-related potentials (ERPs) from the macaque frontal eye field (FEF) to investigate the source of the human N2pc, an attention-related ERP component.

A visual search task was employed where the monkeys made a saccade to a target (L or T) among a group of 1, 3, or 7 distractors (T or L). The target selection time was measured by three spatially distinct electrophysiological techniques; microelectrodes recorded single-neuron spikes and LFPs in the FEF while skull electrodes recorded m-N2pc ERP components over extrastriate visual cortex, hypothesized to be the macaque homologue to the human N2pc. Selection times were consistently faster in single-neuron spikes than LFPs, and LFP selection times were consistently faster than m-N2pc selection times. The selection times in all three techniques increased in parallel with the number of distractors, which is consistent with increased saccade response times. In a majority of test pairs there was a trial-by-trial correlation between the amplitude of FEF LFPs and the amplitude of extrastriate ERPs. These results supported the authors’ hypothesis that “feedback from FEF contributes to the generation of the m-N2pc component.”

There is often a gap between scientific studies performed in humans and those performed in animal models. The advantage of animal models is the ability to invasively explore the underlying biology and physiology of disease and behavior that are, for obvious ethical reasons, otherwise prohibitive in humans. Here, Cohen et al. used two invasive electrophysiological techniques (single-neuron spike recording and LFP) commonly used in animal studies in parallel with one noninvasive electrophysiological technique (ERP) that is frequently used in human studies. These techniques span the spatial scale and together provide a better understanding of the origin of monkey selective visual attention that can in turn be directly utilized in human research. The combination of methodologies provides exceptional temporal and spatial resolution to overcome the limited spatial resolution inherent to EEG scalp recordings. In doing so, the authors succeed in bridging the gap between human and nonhuman primate electro-physiology. One expects to see more investigators employing similar multimodal approaches to further our understanding of the neural bases of noninvasive technologies.