Two periods of processing in the (circum)striate visual cortex as revealed by transcranial magnetic stimulation
Introduction
With the technique of transcranial magnetic stimulation (TMS), it is possible to disrupt the activity in the underlying neural tissue. When the application of TMS over a particular cortical area suppresses the performance of some particular task or function, we can infer that that area is necessary for that task or function (with a spatial resolution of the order of 10 mm). By then looking at the precise timing at which TMS causes suppression, we can also infer when the activity of that area is necessary for that task or function (with a temporal resolution of the order of 1 to 10 ms). Amassian et al. [1]were the first to demonstrate suppression of visual perception with TMS: subjects were unable to identify visually presented letters whenever a TMS pulse was given over the occipital pole between 80 ms and 100 ms after the onset of the presentation of the letter. Although Beckers and Homberg [3]and Beckers and Zeki [4]reported suppression of visual perception at much earlier times in a motion direction discrimination task with TMS over V5, they 3, 4, together with all other investigators 2, 9, 10, 13, 15reported suppression with TMS over the occipital pole only at about the same (later) times as those first reported by Amassian et al. [1]. The aim of our experiments was to achieve suppression of visual perception with TMS over the occipital pole at much earlier times, corroborating cellular recording experiments in primates 6, 7, 11, 14, 16, 20and in humans [21]that show striate cortical activity starting as early as 20 ms [14], 27 ms [16], 30 ms [7], 31 ms [21], 36 ms [11], 41 ms [6], and 20–40 ms [20]after the onset of the visual stimulus. We hoped to achieve this by presenting subjects with small, individual letters confined to the foveal region, at high contrast, for a short time, and with immediate postmasking, which collectively should ensure that the cortical processing has an early onset, a short duration, and a high sensitivity to TMS.
Section snippets
Methods
The main goal of Experiment 1 was to investigate the times at which TMS could suppress the visual identification of a letter when it was presented with very high spatial and temporal limitations. The critical independent variable was the delay (d ), defined as the time between the onset of the visual stimulus and the onset of the magnetic stimulus, ranging from d = −100 ms to d = 190 ms, in steps of 10 ms. In defining the delay, we took into account the 5 ms that it takes the electron beam, at a
Methods
The main goal of Experiment 2 was to show whether (any part of) the wide early dip present in three out of four subjects (s = 2, s = 3, s = 4) was indeed attributable to disruption of activity in the visual cortex. The methods were the same as in Experiment 1 with the following exceptions. First, Experiment 2 had two critical independent variables: the delay (d), defined as the time between the onset of the visual stimulus and the onset of the magnetic stimulus (d = −60 ms, d = −50 ms, d = −10
General Discussion
The results of Experiment 1 corroborated the results of previous investigators 1, 2, 3, 4, 9, 10, 13, 15in that they showed a late dip with the TMS coil placed over the occipital pole. This late dip invariably started at delay d = 60 ms, and descended steeply towards delays d = 90 ms and d = 100 ms, where letter identification was abolished. More importantly, in three out of four subjects, we also found a wide early dip, that invariably ended at delay d = 50 ms, but whose start showed a
Acknowledgements
Erik Corthout was supported by a D. Collen Research Foundation Fellowship. Bob Uttl was supported by the Henry Jackson Foundation. Ulf Ziemann was supported by grant Zi 542/1-1 from the Deutsche Forschungsgemeinschaft.
References (21)
- et al.
Suppression of visual perception by magnetic coil stimulation of human occipital cortex. Electroencephalography and Clinical Neurophysiology
(1989) Impairment of visual perception and visual short term memory scanning by transcranial magnetic stimulation of occipital cortex. Experimental Brain Research
(1991)- Beckers G, Homberg V. Cerebral visual motion blindness: transitory akinetopsia induced by transcranial magnetic...
The consequences of inactivating areas V
(1995)- Bischoff C, Liscic R, Meyer BU, Machetanz J, Conrad B. Magnetically elicited blink reflex: an alternative to...
- et al.
Dynamics of orientation coding in area V
(1993) - Cowey A. Projection of the retina on to the striate and prestriate cortex in the squirrel monkey, Saimiri sciureus....
- et al.
Visual cortical correlates of visible persistence. Vision Research
(1985) Magnetic coil suppression of extrafoveal visual perception using disappearance targets. Journal of Clinical Neurophysiology
(1996)- et al.
Transient visual field defects induced by transcranial magnetic stimulation over human occipital pole. Experimental Brain Research
(1998)