掩蔽刺激对目标识别加工的作用:来自fNIRS的证据

The role of masking stimulation in target recognition processing: Evidence from fNIRS

信息掩蔽刺激(如随机字母群)对字母视觉搜索的干扰大于能量掩蔽刺激(如将字母群像素随机化后的散点),但相应的中枢机理还不清楚。本研究采用记录脑代谢激活模式的功能性近红外光学脑成像技术(fNIRS),考察年轻成年被试分别在字母掩蔽、字母碎片掩蔽、像素掩蔽条件下判断上、下、左、右四个目标字母是否相同时的大脑皮层氧合血红蛋白浓度的变化。结果显示,依字母、字母碎片、随机散点掩蔽条件顺序,被试的搜索任务成绩显著递增,而顶.枕皮层的激活范围与程度显著递减,表明信息掩蔽较匹配的能量掩蔽对视觉搜索有更大的干扰作用,并在初级和联合视觉皮层引发更大的激活。在字母碎片掩蔽条件下,视觉初级皮层部分区域的激活水平与搜索行为绩效的相关显著,而在字母掩蔽条件下,视觉联合皮层部分区域的激活水平与搜索行为绩效的相关显著。这进一步说明信息掩蔽中的字母掩蔽和字母碎片掩蔽的掩蔽作用在大脑皮层上所造成的加工负载存在差异。

When our visual system processes target signals, it usually receives large amounts of irrelevant information from the target, leading to a reduction in the visibility of the target. A wealth of research has shown that visual search for target letters against a masking background is largely determined by the masker type. Informational maskers, such as either randomly positioned and oriented letters or randomly distributed letter fragments, induce stronger masking effects on recognition of target letters than the energetic maskers do, such as the random-phase masker (same spectral amplitude composition as the letter masker but with the phase spectrum randomized) or the random-pixel masker (the locations of the letter maskers’ pixel amplitudes being randomized). However, the mechanisms under informational masking and those under energetic masking are still unknown. The current study examined both cortical activities and behavioral performances in the visual search task, which is determined by whether one of four letters presented at four symmetrically-located positions differs from the others under three masking conditions (random pixels, letter fragments, and random letters). Both the oxygenated hemoglobin concentration (HbO) responses in the primary visual cortex (V1) and secondary visual cortex (V2) with a functional near infrared spectroscopy (fNIRS) were recorded. Twenty (4 males, 16 females) healthy adults (mean age: 22.5 ± 1.67 years) participated in the experiment. Each masking condition contained 5 blocks, and each block contained 8 trails. There was a resting phase of 20 seconds between the two blocks. Spatial registration methods were applied to localize the cortical regions underneath each channel and to define two regions of interest (ROIs), which are the primary visual cortex (V1) and secondary visual cortex (V2). The behavioral results showed that the performance of recognizing target letters improved when the masker type shifted from random letters to letter fragments and to random pixels, suggesting that the letter masker interfered the most with performance than the letter fragment and random-pixel maskers. The random-pixel masker caused the least masking effect. The fNIRS results showed that both letter masker and letter-fragment masker produced an increase in cortical oxygen level. Many regions of interest (ROIs), particularly the visual cortex (including V1 and V2), were more activated under the letter or the letter-fragment masking condition compared to the random-pixel masking condition. Moreover, the differences in cortical activation between the masking conditions further suggested that the V1 and V2 are the critical brain regions involved in visual letter search and informational masking of letter recognition. To summarize, this study used fNIRS to explore the cortex activation patterns of different types of masking on target recognition. The results showed that information masking had much more interference on visual search and caused greater processing loads in primary and secondary visual cortex, compared with energy masking under the same conditions. Furthermore, the differences between letter fragments masking and letters masking are reflected in the activation mode of V1 and V2 regions.