Left Field Stimuli Research Articles

Emotionally arousing stimuli compete for attention with left hemispace

Rapid interaction of the emotional and attentional networks is critical for adaptive behavior. Here, we examined the effects of emotional stimulation on hemifield attention allocation using event-related potential and behavioral measures. Participants performed a visual-discrimination task on nonemotional targets presented randomly in the left or right hemifield. A brief task-irrelevant emotional (pleasant or unpleasant; 150-ms duration) or neutral picture was presented centrally 350 ms before the next target (150-ms duration). Unpleasant stimuli interfered with the left visual field attention capacity, slowing behavioral responses to attended left field stimuli. In keeping with the behavioral data, event-related potential responses to nonemotional attended left field stimuli were reduced over the right parietal regions when preceded by an unpleasant event. The results provide electrophysiological and behavioral evidence that unpleasant, emotionally arousing stimuli interfere with the right hemisphere-dependent attention capacity.

Impaired visual processing of contralesional stimuli in neglect patients: a visual-evoked potential study

Transient visual-evoked potentials (VEPs) were recorded in 11 patients with right brain damage and spatial neglect. High-resolution EEG was recorded using focal stimuli located in the four visual quadrants. VEPs to left stimuli, i.e. located in the neglected side, were compared to VEPs to right stimuli. Results showed that bottom-up processing of a visual stimulus located in the neglected hemifield was intact up to approximately 130 ms from stimulus onset. Hemispheric differences were not significant for either C1 or P1 components representing the activity of striate and extrastriate areas, respectively. In contrast, visual processing in more dorsal areas adjacent to the superior parietal lobe was changed from normal. We failed to record the N1a component for left visual field stimuli expected in the 130-160 ms time range. Furthermore, the N1p (140-180 ms) and P2 (180-220) components were delayed and/or reduced in amplitude for stimuli located on the neglected side. The source of the N1a was previously localized in the intraparietal sulcus in the dorsal occipital cortex; N1p may represent a reactivation of area V3A and P2 reactivation of occipital visual areas including V1 due to top-down feedbacks. Six patients with left brain damage (LBD) and no neglect and 21 healthy subjects were also tested in the same experimental conditions used for patients with neglect. In LBD patients, all components evoked by contralesional stimuli were comparable to ipsilesional components. Overall, data allow localizing in time and space the processing deficit specific for patients with neglect. The first takes place around 130 ms in the bottom-up processing at the level of the anatomically intact dorsal parietal areas; the second is located at the level of the reactivation of the striate and extrastriate areas via feedback connections from higher visual areas. The two functional impairments were limited to left-field stimuli.

Spatial asymmetries of auditory event-synthesis in humans

We used the mismatch negativity event-related potential to examine how spatial location and feature variation affect the capacity of the auditory system to automatically respond to pairs of rapid (180 ms apart) acoustic changes within a single tone. When a tone first deviated from a standard tone in source location and then in its duration, we found independent responses to both deviations for right but not left field stimuli. In contrast, when the first deviation was in pitch and the second in duration, only the first deviation elicited a response, regardless of presentation side. These results suggest that information from either side of space is asymmetrically processed even in a free-field, and that the extent of the temporal window of integration is not a fixed property of the auditory system.

Imagined and actual limb selection: A test of preference

Imagined and actual motor performance were compared to determine what factor(s) drive limb selection for programming movements in contralateral hemispace. Forty right-handed blindfolded subjects were asked to 'reach' via auditory stimulus for a small object placed at multiple locations in hemispace. Two conditions were included: arms uncrossed and arms crossed. With the uncrossed condition, responses were similar. With arms crossed, subjects had the choice of keeping the limbs crossed, reacting to proximity, or uncrossing the arms to reach ipsilaterally. In this condition subjects 'imagined' that they would maintain the crossedposition and reach with the hand closest to the stimulus in both right and left hemispace. However, during 'actual' reaching, responses differed. For left-field stimuli, participants kept the arms crossed, but in response to right-field stimuli, subjects preferred to uncross the limbs in order to reach with the dominant hand. These findings suggest that while motor dominance is the primary factor in limb choice for action in ipsilateral hemispace, it appears that object proximity drives limb selection for reaching in contralateral hemispace.

Intra-modal and cross-modal spatial attention to auditory and visual stimuli. An event-related brain potential study

This study investigated cross-modal interactions in spatial attention by means of recording event-related brain potentials (ERPs). Noise bursts and light flashes were presented in random order to both left and right field locations separated by 60° in free-field. One group of subjects was instructed to attend selectively to the noise bursts (attend-auditory group), and a second group attended only to the flashes (attend-visual group). On different runs attention was directed to either the right or left field stimuli of the designated modality. In the attend-auditory group, noise bursts at the attended location elicited a broad, biphasic negativity (Nd) beginning at 70 ms. The cross-modal spatial attention effect on the auditory ERPs in the attend-visual group was very similar in morphology, but the Nd was reduced in amplitude relative to the intra-modal effect. In the attend-visual group, flashes at the attended location elicited enhanced early (100–200 ms) and late (200–350 ms) ERP components relative to unattended-location flashes. The cross-modal effect in the attend-auditory group included small but significant enhancements of early components of the visual ERPs. It was concluded that spatial attention has a multi-modal organization such that the processing of stimuli at attended locations is facilitated at an early, sensory level, even for stimuli of an unattended modality.

Dissociation of Hemispheric Exploitation of Rods and Cones for Simple Detection

Sergent (1982a, 1982b, 1982c) proposed that stimuli carrying high luminous energy will be better detected in the right field (left hemisphere) and stimuli carrying low energy in the left visual field (right hemisphere). A photoreceptor-based model could explain the same effects as well as several others (eccentricity effects, retinal adaptation effects). Braun, Mailloux, and Dufresne (1995) suggested that cones might favor right field stimuli and rods might favor left field stimuli. We implemented a series of near-threshold simple detection experiments comprising stimuli varying along four dimensions (color, eccentricity, duration, and size) in dark-adapted subjects. Consistent and significant right field advantages accrued for near-meridianal foveal red stimuli, but there were no field effects for eccentric blue stimuli. An alternative to Sergent's stimulus energy model and the photoreceptor-based model, namely a parvocellular-magnocellular model, is proposed.

Visual evoked potentials to stimuli in apparent motion

Following an extensive single subject pilot study 12 Ss viewed a continuously cycling (3 Hz), two stimulus, apparent motion (AM) display. The “AM breakdown effect” was utilised to gather visual evoked potentials (VEPs) from sites OOl, O2, T5, T6, P3, and P4 during periods in which the display elicited either: the percept of motion; or the percept of two discrete alternating stimuli. VEPs displayed components of larger amplitude during motion periods in all right (but no left) hemisphere sites during periods 60–82 msec post left field stimulus onset (positive component) and 100–126 msec post right field stimulus onset (negative component). Results were interpreted as indicating initial extracion of motion information within the occipital lobe, with further motion processing taking place in temporal and parietal lobes.

Information processing of visual stimuli in an "extinguished" field.

Lesions of the right parieto-occipital cortex in man produce a variety of behavioural disturbances which interfere with the detection of, and orientation to external stimuli1. One striking example is extinction to double simultaneous stimulation (DSS), in which presentation of a single stimulus in any area of the visual field results in its accurate description, but lateralised simultaneous presentation of two stimuli, one in each field, results in the verbal description of only the stimulus in the right visual field (RVF)2. While extinction to DSS can also be demonstrated in other (non-visual) sensory modalites3, and is occasionally seen following left-hemisphere lesions4 our concern here is with visual extinction to DSS following right parieto-occipital lesions whose precise nature is poorly understood although variety of theories have been proposed to account for it5–7. One of the critical, yet unexplored questions about extinction concerns the fate of the extinguished stimulus. The following observations demonstrate that although the extinguished stimulus often goes completely unnoticed by the patient, the patients are able to utilise the extinguished stimulus in an interfield comparison task. Accurate same/different judgments between the visual fields could be made in situations in which the patients did not know the identity of, and at times even denied the presence of, the left field stimulus.

Interhemispheric interaction during simultaneous bilateral presentation of letters or digits in commissurotomized patients

Interhemispheric interaction after forebrain commissurotomy was studied in six patients. Letters or digits were flashed for 0·1 sec either in the left or in the right visual field alone, or in both fields simultaneously. Patients were asked to identify the stimuli either verbally or by hand. Results showed generally better performance for each hemisphere during unilateral than during bilateral stimulus presentation. For both verbal and manual responses, identification of right-field stimuli was better than that of left-field stimuli, the latter often being completely ignored during bilateral presentations. Some evidence was obtained for right-hemisphere verbalization during unilateral presentation of left-field stimuli.

Retinal Locus in Tachistoscopic Binocular Color Rivalry

Tachistoscopic binocular color rivalry was studied for stimuli either centrally fixated or displaced into the left or the right visual field. The distribution of two-color configurations did not differ markedly among the three locations, but the relative proportion of the percept seen in the color presented to the right eye was less for the left field stimuli than for foveal stimuli or right-field stimuli.