The Effect of Saccades on Dorsal- and Ventral-Stream Visual Processing

Some cognitive processes are suppressed during saccadic eye movements, whereas others are not. In two experiments, we investigated the locus of this interference effect. In one experiment, subjects decided whether pictured items were objects or nonobjects while making saccades of different lengths. Saccade distance had no effect on response time, indicating that saccades do not interfere with object recognition. However, in a second experiment, in which subjects decided whether pictured items faced to the left or to the right, response time increased with saccade distance, indicating that processing was suppressed during the saccade. These results (along with others) suggest that dorsal-stream (where) processes are suppressed during saccades, whereas ventral-stream (what) processes are not. Because the dorsal stream is instrumental in generating saccades, we propose that cognitive saccadic suppression results from dual-task interference within this visual subsystem. People make rapid, saccadic eye movements to change the locus of fixation three or four times each second when they read, view pictures, or explore the world around them (Rayner, 1978, 1998). This means that saccades are among the most frequent behaviors that people perform, taking place 180 times each minute, 10,800 times each hour, and 172,800 times each 16-hr waking day. Assuming an average saccade duration of 30 ms, the eyes are in motion about 90 min each day. During saccadic eye movements, sensitivity to visual input is reduced, such that visual perception is confined to fixations—a phenomenon usually called saccadic suppression (Matin, 1974; Zuber & Stark, 1966). Thus, 90 min during the day when people think they are seeing, they actually are not. Recently, several studies have shown that some cognitive processes are also disrupted during saccades, raising the possibility that 90 min during the day when people think they are thinking, they actually are not. The idea that cognitive processing is suppressed during saccades may seem very implausible because people are not aware of pauses in mental activity during eye movements. Saccade durations are typically very brief, however, so any disruptions that might occur may go unnoticed, just as the disruptions in visual input that accompany saccades and eyeblinks are usually unnoticed. A variety of cognitive tasks have been shown to be suspended during saccadic eye movements. Sanders and his colleagues have demonstrated that stimulus encoding, the resolution of degraded stimuli, and memory scanning are suppressed during saccades (Sanders & Houtmans, 1985; Sanders & Rath, 1991; Van Duren, 1993). In addition, Matin, Shao, and Boff (1993) reported that processing time in a counting task increases when eye movements must be made to acquire information. More recently, we have found that mental rotation is suppressed during saccades (Irwin & Brockmole, 2000; Irwin & Carlson-Radvansky, 1996), as are changes in attentional scale from local to global levels of a form (Brockmole, Carlson, & Irwin, 2002). Other studies, however, have shown that not all cognitive processes are suppressed during saccades. For example, Van Duren and Sanders (1995) reported that saccades do not interfere with response selection, and we have found that identity priming (Irwin, Carlson-Radvansky, & Andrews, 1995) and word recognition and word identification (Irwin, 1998) continue normally during saccades. Thus, some mental operations are suppressed during saccades, whereas others are not. In this article, we propose that the distinction between these processes may correspond to the now well-known distinction between the what and where cortical processing streams proposed by Ungerleider and Mishkin in the 1980s (e.g., Mishkin, Ungerleider, & Macko, 1983). They proposed that two functionally separable streams of visual processing exist in the brain. The ventral stream projects to areas of inferotemporal cortex and is hypothesized to be responsible for perceptual recognition and cognitive representations of objects; ventral-stream processing is involved in identity priming and word recognition, which are not suppressed during saccades. The dorsal stream projects to the posterior parietal cortex and is hypothesized to be responsible for visuospatial operations, including mental rotation and shifts of visual attention, which are disrupted by saccades. Although more recent evidence has called into question the separability of the dorsal and ventral pathways (e.g., Merigan & Maunsell, 1993), as well as the interpretation of their function (e.g., Goodale & Milner, 1992), this division continues to have useful heuristic value. For present purposes, for example, it suggests a possible explanation for why some cognitive processes are suppressed during saccades while others are not. Saccade generation and saccade execution rely Address correspondence to David E. Irwin, Department of Psychology, University of Illinois, 603 E. Daniel St., Champaign, IL 61820; e-mail: dirwin@s.psych.uiuc.edu. PSYCHOLOGICAL SCIENCE Volume 15—Number 7 467 Copyright r 2004 American Psychological Society on the dorsal stream, especially the parietal cortex (e.g., Schall, 1995). Therefore, the execution of saccades during the execution of cognitive processes that also rely on the dorsal stream might result in dual-task interference and the disruption of cognitive processing. Even though saccades are commonplace and generally go unnoticed, it is nonetheless the case that whenever people are engaged in some task and moving their eyes, they are in a dual-task situation. In dual-task situations, interference generally occurs when both tasks need the same processing structures in order to be executed (e.g., Meyer & Kieras, 1997; Pashler, 1984). Thus, competition would exist between saccades and dorsal-stream processes, but not between saccades and cognitive processes that engage the ventral stream. This hypothesis is generally consistent with the results of previous research, but is based on a comparison of several different tasks involving different stimuli, different instructions, different settings, and so on. The present research directly tested this hypothesis by having subjects perform different tasks on the same set of stimuli, namely, pictures of objects. Some subjects performed a ventral-stream task on these stimuli (object recognition), whereas others performed a dorsalstream task (object orientation judgment). We predicted that saccades would interfere with the dorsal-stream task only. EXPERIMENT 1: OBJECT RECOGNITION DURING