Reexamining Visual Orientation Anisotropies: A Bias Towards Simple Horizontal Stimuli on Temporal Order Judgments

Reexamining Visual Orientation Anisotropies: A Bias Towards Simple Horizontal Stimuli on Temporal Order Judgments Ahnate Lim (ahnate@hawaii.edu) Department of Psychology, University of Hawaii at Manoa 2530 Dole Street, Honolulu, HI 96822 USA Scott Sinnett (ssinnett@hawaii.edu) Department of Psychology, University of Hawaii at Manoa 2530 Dole Street, Honolulu, HI 96822 USA Abstract Although not currently a widely accepted notion, evidence suggests an anisotropy between horizontal and vertical orientations in visual processing. While there is evidence of an early neurological bias due to a greater number of cortical neurons tuned to the horizontal orientation, recent behavioral evidence suggests a “horizontal effect”, where performance for broadband horizontal stimuli is worse compared to vertical and oblique. Importantly however, this effect has only been observed for complex stimuli and is speculated to counterbalance for the greater occurrence of horizontal stimuli in the environment. In this experiment, we used a staircase temporal order judgment task in three spatial configurations (horizontal, vertical, and both) to test for 1) a bias towards either horizontal or vertical simple stimuli, and 2) whether performance would vary across different planes of stimuli presentation. A bias towards horizontal stimuli was observed, but only when presented in the horizontal plane. Theoretical implications are discussed. Keywords: Horizontal bias; visual processing; anisotropy, stimulus orientation; temporal order judgment Introduction Research has shown that in humans as well as many other species, visual (and tactile) stimuli are processed differently depending on their orientation. One of the more commonly observed biases in visual perception is a phenomenon termed the “oblique effect”, where stimuli presented in an oblique orientation are usually processed worse (i.e., in speeded detection, identification, resolution acuity and contrast sensitivity tests) than stimuli presented in the horizontal or vertical position (Appelle, 1972; Essock, 1980; for tactile, see Essock, Krebs, & Prather, 1997). Crucially, this phenomenon operates on at least two different levels. First, it has been linked causally to lower level vision, where it is attributed to differences in the number of cortical neurons in V1 tuned to stimulus orientation (Anzai, Bearse, Freeman, & Cai, 1995). Secondly, the effect also appears to be manifested in higher level cognitive processes such as memory, learning, and perception (for review, see Essock, 1980). The distinction between these two levels has in fact led to their classification as Class 1 and Class 2 oblique effects, respectively (Essock, 1980). Apart from the well known oblique effect, studies that have attempted to compare anisotropies of horizontal and vertical orientations themselves against each other may not have done so carefully (Hansen & Essock, 2004). It is worth noting that the current prevailing viewpoint is that horizontal and vertical stimuli are treated equally at the physiological level. Notwithstanding this dogma, there is considerable evidence suggesting that there is more neural circuitry in the visual cortex devoted to processing horizontal contours than vertical contours (Chapman & Bonhoeffer, 1998 (Figs. 1 and 2); Chapman, Stryker, & Bonhoeffer, 1996 (Figs. 1 and 2); Coppola, White, Fitzpatrick, & Purves, 1998; Mansfield, 1974; Mansfield & Ronner, 1978; Tiao & Blakemore, 1976). For instance, a study examining a large database of neurons in the cat’s striate cortex found that the largest population of cells are activated by orientations close to the horizontal position (Li, Peterson, & Freeman, 2003). Accordingly, it is curious that such a horizontal over vertical preference has not correspondingly been observed in behavioral tasks. In fact, and despite the seeming neurological advantage for processing stimuli in the horizontal orientation, a study by Essock, DeFord, Hansen, and Sinai (2003) recently found diminished behavioral performance for horizontally presented stimuli (termed the “horizontal effect”). Furthermore, with complex “realistic” stimuli, they found that perceived orientation for broadband spatial content using horizontal, vertical, and oblique gratings was actually lowest for horizontal gratings, while oblique was instead seen best—a result seemingly contrary to the oblique effect, but solely at face value since the horizontal effect only appears to operate on complex stimuli. Interestingly, they explain these robust effects as being possibly due to a “whitening” mechanism that decreases the saliency of horizontal stimuli (which is argued to be most prevalent in natural scenes), thereby increasing the saliency of other broad-spectrum objects (such as predators, for instance). Further research by Hansen and Essock (2004) replicated these findings in an experiment that used both simple and more complex “realistic” gratings. The classic “oblique effect” was seen with the simple gratings, whereas a “horizontal effect” (similar to Essock et al., 2003) was observed with the complex gratings. Additionally, the authors conducted an aggregate analysis of various natural scenes and found the prevalence of stimuli orientation in these scenes to be most prevalent in the horizontal orientation, then vertical, with the least prevalent being