Giorgio Ganis

Neurophysiological evidence for visual perceptual categorization of words and faces within 150 ms

The nature and early time course of the initial processing differences between visually matched linguistic and nonlinguistic images were studied with event-related potentials (ERPs). The first effect began at 90 ms when ERPs to written words diverged from other objects, including faces. By 125 ms, ERPs to words and faces were more positive than those to other objects, effects identified with the P150. The amplitude and scalp distribution of P150s to words and faces were similar. The P150 seemed to be elicited selectively by images resembling any well-learned category of visual patterns. We propose that (a) visual perceptual categorization based on long-term experience begins by 125 ms, (b) P150 amplitude varies with the cumulative experience people have discriminating among instances of specific categories of visual objects (e.g., words, faces), and (c) the P150 is a scalp reflection of letterstring and face intracranial ERPs in posterior fusiform gyrus.

The search for “common sense”: An electrophysiological study of the comprehension of words and pictures in reading

Event-related brain potentials (ERPs) from 26 scalp sites were used to investigate whether or not and, if so, the extent to which the brain processes subserving the understanding of imageable written words and line drawings are identical. Sentences were presented one word at a time to 28 undergraduates for comprehension. Each sentence ended with either a written word (regular sentences) or with a line drawing (rebus sentences) that rendered it semantically congruous or semantically incongruous. For half of the subjects regular and rebus sentences were randomly intermixed whereas for the remaining half the regular and rebus sentences were presented in separate blocks (affording within-subject comparisons in both cases). In both presentation formats, words and line drawings generated greater negativity between 325 and 475 msec post-stimulus in ERPs to incongruous relative to congruous sentence endings (i.e., an N400-like effect). While the time course of this negativity was remarkably similar for words and pictures, there were notable differences in their scalp distributions; specifically, the classic N400 effect for words was larger posteriorly than it was for pictures. The congruity effect for pictures but not for words was also associated with a longer duration (lower frequency) negativity over frontal sites. In addition, under the mixed presentation mode, the N400 effect peaked about 30 msec earlier for pictures than for words. All in all, the data suggest that written words and pictures when they terminate sentences are processed similarly, but by at least partially nonoverlapping brain areas.

An electrophysiological study of scene effects on object identification.

The meaning of a visual scene influences the identification of visual objects embedded in it. We investigated the nature and time course of scene effects on object identification by recording event-related brain potentials (ERPs) and response times (RTs). In three experiments, participants identified objects within a scene that were either semantically congruous (e.g., a pot in a kitchen) or incongruous (e.g., a desk in a river). As expected, RTs were faster for congruous than incongruous objects. The earliest sign of reliable scene congruity effects in the ERPs (greater positivity for congruous pictures between 300 and 500 ms) was around 300 ms. Both the morphology and time course of the N390 scene congruity effect are reminiscent of the N400 sentence congruity effect typically observed in sentence context paradigms, suggesting a functional similarity of the neural processes involved. Overall, these results support theories postulating that visual scenes do not appreciably affect object identification processes before associated semantic information is activated. We speculate that the N390 scene congruity effect reflects the action of visual scene schemata stored in the anterior temporal lobe.

Training Generalized Spatial Skills

Spatial transformation skills are an essential aspect of cognitive ability. These skills can be improved by practice, but such improvement has usually been specific to tasks and stimuli. The present study investigated whether intensive long-term practice leads to change that transcends stimulus and task parameters. Thirty-one participants (14 male, 17 female) were tested on three cognitive tasks: a computerized version of the Shepard-Metzler (1971) mental rotation task (MRT), a mental paper-folding task (MPFT), and a verbal analogies task (VAT). Each individual then participated in daily practice sessions with the MRT or the MPFT over 21 days. Postpractice comparisons revealed transfer of practice gains to novel stimuli for the practiced task, as well as transfer to the other, nonpracticed spatial task. Thus, practice effects were process based, not instance based. Improvement in the nonpracticed spatial task was greater than that in the VAT; thus, improvement was not merely due to greater ease with computerized testing.

Adaptive filtering to reduce global interference in non-invasive NIRS measures of brain activation: How well and when does it work?

In previous work we introduced a novel method for reducing global interference, based on adaptive filtering, to improve the contrast to noise ratio (CNR) of evoked hemodynamic responses measured non-invasively with near infrared spectroscopy (NIRS). Here, we address the issue of how to generally apply the proposed adaptive filtering method. A total of 156 evoked visual response measurements, collected from 15 individuals, were analyzed. The similarity (correlation) between measurements with far and near source-detector separations collected during the rest period before visual stimulation was used as indicator of global interference dominance. A detailed analysis of CNR improvement in oxy-hemoglobin (O(2)Hb) and deoxy-hemoglobin (HHb), as a function of the rest period correlation coefficient, is presented. Results show that for O(2)Hb measurements, 66% exhibited substantial global interference. For this dataset, dominated by global interference, 71% of the measurements revealed CNR improvements after adaptive filtering, with a mean CNR improvement of 60%. No CNR improvement was observed for HHb. This study corroborates our previous finding that adaptive filtering provides an effective method to increase CNR when there is strong global interference, and also provides a practical way for determining when and where to apply this technique.

Lying in the scanner: covert countermeasures disrupt deception detection by functional magnetic resonance imaging.

Functional magnetic resonance imaging (fMRI) studies have documented differences between deceptive and honest responses. Capitalizing on this research, companies marketing fMRI-based lie detection services have been founded, generating methodological and ethical concerns in scientific and legal communities. Critically, no fMRI study has examined directly the effect of countermeasures, methods used by prevaricators to defeat deception detection procedures. An fMRI study was conducted to fill this research gap using a concealed information paradigm in which participants were trained to use countermeasures. Robust group fMRI differences between deceptive and honest responses were found without, but not with countermeasures. Furthermore, in single participants, deception detection accuracy was 100% without countermeasures, using activation in ventrolateral and medial prefrontal cortices, but fell to 33% with countermeasures. These findings show that fMRI-based deception detection measures can be vulnerable to countermeasures, calling for caution before applying these methods to real-world situations.

Frontostriatal circuits are necessary for visuomotor transformation : Mental rotation in Parkinson's disease

The mental rotation of objects requires visuospatial functions mediated by the parietal lobes, whereas the mental rotation of hands also engages frontal motor-system processes. Nondemented patients with Parkinsons disease (PD), a frontostriatal disorder, were predicted to be impaired on mentally rotating hands. Side of PD motor symptom onset was investigated because the left motor cortices likely have a causal role in hand mental rotation. The prediction was that patients with right-side onset (RPD, greater left-hemisphere dysfunction) would commit more errors rotating hands than patients with left-side onset (LPD). Fifteen LPD, 12 RPD, and 13 normal control adults (NC) made same/different judgments about pairs of rotated objects or hands. There were no group differences with objects. When rotating hands, RPD, but not LPD, made more errors than the NC group. A control experiment evaluated whether visual field of presentation explained differences between PD subgroups. In the first experiment (1A), the hand to be mentally rotated was presented in the right visual field, but here (1B) it was presented in the left visual field. Only the LPD group made more errors than the NC group. The evidence suggests a double dissociation for the RPD and LPD groups between tasks differing in visual-field presentation. The findings indicate that hemifield location of a to-be-rotated hand stimulus can cause the hemispheric frontoparietal networks to be differentially engaged. Moreover, frontostriatal motor systems and the parietal lobes play a necessary role during the mental rotation of hands, which requires integrating visuospatial cognition with motor imagery.

Visual cortex excitability increases during visual mental imagery--a TMS study in healthy human subjects.

Previous neuroimaging studies provided evidence that visual mental imagery relies, in part, on the primary visual cortex. We hypothesized that, analogous to the finding that motor imagery increases the excitability of motor cortex, visual imagery should increase visual cortex excitability, as indexed by a decrease in the phosphene threshold (PT). In order to test visual cortex excitability, the primary visual cortex was stimulated with transcranial magnetic stimulation (TMS), so as to elicit phosphenes in the right lower visual quadrant. Subjects performed a visual imagery task and an auditory control task. We applied TMS with increasing intensity to determine the PT for each subject. Independent of the quadrant in which subjects placed their visual images, imagery decreased PT compared to baseline PT; in contrast, the auditory task did not change PT. These findings demonstrate for the first time a short-term, task-dependent modulation of PT. These results constitute evidence that early visual areas participate in visual imagery processing.

Repetitive transcranial magnetic stimulation of human area MT/V5 disrupts perception and storage of the motion aftereffect

Following adaptation to a moving stimulus, the introduction of a stationary pattern creates the illusion of motion. This phenomenon, known as the motion aftereffect (MAE), can be delayed by placing a blank storage interval between the adapting and test stimuli. Human motion selective area MT/V5 has been proposed as the likely neural origin of MAEs. To examine the role of MT/V5 in perceiving and storing MAEs, we applied repetitive transcranial magnetic stimulation (rTMS) to this area during a 10s storage interval and while subjects perceived illusory motion. Our results show that rTMS disrupts perception of the MAE when it is delivered in the early parts of the storage period and when it is applied during the perceptual MAE itself. Stimulation of control regions corresponding to V1 or Cz did not affect the MAE. In addition, magnetic stimulation of dorsolateral prefrontal and posterior parietal cortices did not disrupt MAE perception. These data provide experimental support for the notion that MT/V5 subserves perception and storage of the motion aftereffect.

Visual mental imagery and perception produce opposite adaptation effects on early brain potentials

Event-related potentials (ERPs) were recorded during a rapid adaptation paradigm to determine whether visual perception and visual mental imagery of faces recruit the same early perceptual processes. The early effect of face and object adaptors, either perceived or visualized, on test stimuli, was assessed by measuring the amplitude of the N170/VPP complex, typically much larger for faces than for other object categories. Faces elicited a robust N170/VPP complex, localized to posterior ventrolateral occipitotemporal cortex. Both visualized and perceived adaptors affected the N170/VPP complex to test faces from 120 ms post-stimulus, reflecting effects on neural populations supporting early perceptual face categorization. Critically, while perceived adaptors suppressed the amplitude of the N170/VPP, visualized adaptors enhanced it. We suggest that perceived adaptors affect neural populations in the neocortex supporting early perceptual processing of faces via bottom-up mechanisms, whereas visualized adaptors affect them via top-down mechanisms. Similar enhancement effects were found on the N170/VPP complex to non-face objects, suggesting such effects are a general consequence of visual imagery on processing of faces and other object categories. These findings support image-percept equivalence theories and may explain, in part, why visual percepts and visual mental images are not routinely confused, even though both engage similar neural populations in the visual system.