Age-related changes in visually evoked electrical brain activity

Whereas much is known about the degenerative effects of aging on cortical tissue, less is known about how aging affects visually evoked electrical activity, and at what latencies. We compared visual processing in elderly and young controls using a visual masking paradigm, which is particularly sensitive to detect temporal processing deficits, while recording EEG. The results show that, on average, elderly have weaker visual evoked potentials than controls, and that elderly show a distinct scalp potential topography (microstate) at around 150 ms after stimulus onset. This microstate occurred irrespective of the visual stimulus presented. Electrical source imaging showed that the changes in the scalp potential resulted from decreased activity in lateral occipital cortex and increases in fronto-parietal areas. We saw, however, no evidence that increased fronto-parietal activity enhanced performance on the discrimination task, and no evidence that it compensated for decreased posterior activity. Our results show qualitatively different patterns of visual evoked potentials (VEPs) in the elderly, and demonstrate that increased fronto-parietal activity arises during visual processing in the elderly already between 150 and 200 ms after stimulus onset. The microstate associated with these changes is a potential diagnostic tool to detect age-related cortical changes. Hum Brain Mapp, 2011. (c) 2011 Wiley-Liss, Inc.

Transcranial Magnetic Stimulation of Early Visual Cortex Reveals a Window of Integration of Substantial Duration.

Michael Herzog, Johannes Rüter, Frank Scharnowski

How the brain achieves integration of temporally dispersed information is one of the enigmas in the neurosciences. By combining feature fusion with transcranial magnetic stimulation (TMS), we show that individual visual features are independently stored for several hundred milliseconds before condensed in a conscious percept. We presented two verniers with opposite offset directions in rapid succession at the same retinotopic location. Because of the short durations (30ms each) only one fused vernier is perceived. The perceived offset of the fused vernier is a combination of the offsets of the two verniers which themselves are consciously not accessible. First, we balanced performance at approximately 50%, so that neither the first nor the second vernier dominated the fused percept. We then applied TMS at different times after the onset of the first vernier over the occipital cortex to interfere with visual processing. When TMS was applied from 45ms to 120ms after the onset of the first vernier, the second vernier dominated. For later onset asynchronies of up to 370ms, the first vernier dominated. Hence, even though the individual verniers are not consciously accessible, they still can be manipulated by TMS for a period of more than 300ms. Our results demonstrate that the individual verniers are stored independently for several hundred milliseconds before they are condensed into a conscious percept. Thus, the brain collects and integrates feature information across a broad time window before consciousness. Acknowledgement: This work was supported by the SNSF (Swiss National Science Foundation).

2007

Electrophysiological Evidence for Ventral Stream Deficits in Schizophrenia Patients

Michael Herzog, Gijs Plomp, Maya Roinishvili

Schizophrenic patients suffer from many deficits including visual, attentional, and cognitive ones. Visual deficits are of particular interest because they are at the fore-end of information processing and can provide clear examples of interactions between sensory, perceptual, and higher cognitive functions. Visual deficits in schizophrenic patients are often attributed to impairments in the dorsal (where) rather than the ventral (what) stream of visual processing. We used a visual-masking paradigm in which patients and matched controls discriminated small vernier offsets. We analyzed the evoked electroencephalography (EEG) responses and applied distributed electrical source imaging techniques to estimate activity differences between conditions and groups throughout the brain. Compared with controls, patients showed strongly reduced discrimination accuracy, confirming previous work. The behavioral deficits corresponded to pronounced decreases in the evoked EEG response at around 200 ms after stimulus onset. At this latency, patients showed decreased activity for targets in left parietal cortex (dorsal stream), but the decrease was most pronounced in lateral occipital cortex (in the ventral stream). These deficiencies occurred at latencies that reflect object processing and fine shape discriminations. We relate the reduced ventral stream activity to deficient top-down processing of target stimuli and provide a framework for relating the commonly observed dorsal stream deficiencies with the currently observed ventral stream deficiencies.

Oxford Univ Press2013

Neural correlates of non-retinotopic visual processing and light adaptation

Yvonne Evelina Thunell

In the main part of this thesis, I investigate the neural correlates of nonretinotopic processing. We might intuitively think of the visual representation of the world as retinotopic, as is the case for the organization of most visual brain areas, but in fact our perception is usually not retinotopic. Consider for example a bicycle passing by with a reflector on one of its wheels. The reflector appears to move in a circular or prolate cycloidal orbit. It’s true motion trajectory, however, is a curtate cycloid. It seems that we perceive the reflector motion relative to the bicycle. The bicycle thus serves as a moving reference system for computing the motion of its constituent parts. Only little is known about the neural correlates of this type of non-retinotopic processing. Here, using EEG, I investigate the temporal dynamics of the encoding of non-retinotopic motion relative to a moving reference system. The results indicate that the motion of the reference system is discounted from the motion of the part already from 120 ms after stimulus onset and that the non-retinotopic representation dominates throughout the rest of the visual processing. Further, I use fMRI to show that the earliest area in the visual hierarchy holding non-retinotopic representations is the human motion processing complex (hMT+). In the second part of the thesis, the EEG correlates of light adaptation are investigated. Light adaptation is crucial for coping with the large range of ambient luminance values in our environment. We first confirm the known adaptation effect; the overall response to light flashes decreases substantially after adaption to bright light as compared to dim light. Furthermore, this adaptation effect depends on the wavelength of the light, being most pronounced for red light as compared to white, green and blue light. In addition, at short latencies (less than 100 ms) responses to light flashes are stronger after bright than after dim light adaptation for all colors except red. Our results show that even very short-latency EEG responses are differentially affected by different wavelengths of light.

EPFL2014

Neural correlates of non-retinotopic visual processing and light adaptation

Yvonne Evelina Thunell

EPFL2014