Degg C, Slaven A, Armstrong R A
Vision Sciences, Aston University, Birmingham, UK.
Brain Topogr. 1992 Fall;5(1):11-6. doi: 10.1007/BF01129965.
The topography of the visual evoked magnetic response (VEMR) to a pattern onset stimulus was investigated using 4 check sizes and 3 contrast levels. The pattern onset response consists of three early components within the first 200ms, CIm, CIIm and CIIIm. The CIIm is usually of high amplitude and is very consistent in latency within a subject. Half field (HF) stimuli produce their strongest response over the contralateral hemisphere; the RHF stimulus exhibiting a lower positivity (outgoing field) and an upper negativity (ingoing field), rotated towards the midline. LHF stimulation produced the opposite response, a lower negative and an upper positive. Larger check sizes produce a single area of ingoing and outgoing field while smaller checks produce on area of ingoing and outgoing field over each hemisphere. Latency did not appear to vary with change in contrast but amplitudes increased with increasing contrast. A more detailed topographic study incorporating source localisation procedures suggested a source for CIIm-4cm below the scalp, close to the midline with current flowing towards the lateral surface. Similar depth and position estimates but with opposite polarity were obtained for the pattern shift P100m previously. Hence, the P100m and the CIIm may originate in similar areas of visual cortex but reveal different aspects of visual processing.
使用4种方格大小和3种对比度水平，研究了对视标起始刺激的视觉诱发电磁反应（VEMR）的地形图。视标起始反应在前200毫秒内由三个早期成分组成，即CIm、CIIm和CIIIm。CIIm通常具有高幅值，并且在受试者内潜伏期非常一致。半视野（HF）刺激在对侧半球产生最强反应；右半视野（RHF）刺激表现出较低的正性（外向场）和较高的负性（内向场），并向中线旋转。左半视野（LHF）刺激产生相反的反应，较低的负性和较高的正性。较大的方格大小产生单个内向和外向场区域，而较小的方格在每个半球上产生内向和外向场区域。潜伏期似乎不随对比度变化而变化，但幅值随对比度增加而增大。一项纳入源定位程序的更详细的地形图研究表明，CIIm的源位于头皮下方4厘米处，靠近中线，电流流向外侧表面。先前对图形移位P100m获得了类似的深度和位置估计，但极性相反。因此，P100m和CIIm可能起源于视觉皮层的相似区域，但揭示了视觉处理的不同方面。
