Primate frontal eye fields. II. Physiological and anatomical correlates of electrically evoked eye movements

Abstract

We studied single neurons in the frontal eye fields of awake macaque monkeys and compared their activity with the saccadic eye movements elicited by microstimulation at the sites of these neurons. Saccades could be elicited from electrical stimulation in the cortical gray matter of the frontal eye fields with currents as small as 10 microA. Low thresholds for eliciting saccades were found at the sites of cells with presaccadic activity. Presaccadic neurons classified as visuomovement or movement were most associated with low (less than 50 microA) thresholds. High thresholds (greater than 100 microA) or no elicited saccades were associated with other classes of frontal eye field neurons, including neurons responding only after saccades and presaccadic neurons, classified as purely visual. Throughout the frontal eye fields, the optimal saccade for eliciting presaccadic neural activity at a given recording site predicted both the direction and amplitude of the saccades that were evoked by microstimulation at that site. In contrast, the movement fields of postsaccadic cells were usually different from the saccades evoked by stimulation at the sites of such cells. We defined the low-threshold frontal eye fields as cortex yielding saccades with stimulation currents less than or equal to 50 microA. It lies along the posterior portion of the arcuate sulcus and is largely contained in the anterior bank of that sulcus. It is smaller than Brodmann's area 8 but corresponds with the union of Walker's cytoarchitectonic areas 8A and 45. Saccade amplitude was topographically organized across the frontal eye fields. Amplitudes of elicited saccades ranged from less than 1 degree to greater than 30 degrees. Smaller saccades were evoked from the ventrolateral portion, and larger saccades were evoked from the dorsomedial portion. Within the arcuate sulcus, evoked saccades were usually larger near the lip and smaller near the fundus. Saccade direction had no global organization across the frontal eye fields; however, saccade direction changed in systematic progressions with small advances of the microelectrode, and all contralateral saccadic directions were often represented in a single electrode penetration down the bank of the arcuate sulcus. Furthermore, the direction of change in these progressions periodically reversed, allowing particular saccade directions to be multiply represented in nearby regions of cortex.(ABSTRACT TRUNCATED AT 400 WORDS)

Topographic organization of macaque area LIP

“…Previous electrophysiology studies placed the fovea dorsal to the peripheral field representation and concluded that LIP contained a coarse but contiguous retinotopic map of the contralateral field (3,7,14). However, our results are consistent with previous imaging studies reporting a rostral foveal representation (14,21) and with LIP/frontal eye field (FEF) connectivity studies demonstrating that rostral LIP projects to the ventral portion of FEF representing the fovea/parafovea and caudal LIP to the dorsal portion of FEF representing the periphery (2,35,36). Differences between our results and those of the previous electrophysiology studies that used fixation to define the foveal representation in LIP may actually reflect a separation of neurons involved in fixation from those involved in the processing of foveal stimuli (14).…”

Spatial maps in frontal and prefrontal cortex

“…Spatial maps related to eye movements, spatial attention, and working memory have been identified in monkey frontal and prefrontal cortex, in and around the FEF (Robinson and Fuchs, 1969;Suzuki and Azuma, 1983;Bruce et al, 1985) and principal sulcus (Sawaguchi and Iba, 2001;Roe et al, 2004). It has been argued that the human homologues of both the FEF and the principal sulcus area are located more superiorly than in monkeys (Courtney et al, 1998), suggesting that our DLPFC focus may be comparable to a more region inferior to the FEF and the principal sulcus in monkeys.…”

Cortical activations during paced finger-tapping applying visual and auditory pacing stimuli

“…It is located at the the control of the visual and auditory tasks. ITI variability level of the frontal eye-fields as defined by microstimulaas measured by the coefficient of variation (S.D.3100 / tion [3]. In PET studies, the frontal eye-fields have been mean ITI) was also subjected to a two-way analysis of identified as lying from 40 to 56 mm above the AC-PC variance with repeated measurements on both factors.…”