Who's got the global advantage? Visual field differences in processing of global and local shape

Abstract

Much evidence suggests that we first perceive the overall layout of a scene or object followed later by the details. This coarse-to-fine temporal dynamic in visual processing is also found in Navon's classical paradigm where information at the global level of compound stimuli is processed faster than information at the local level (global precedence effect), and where information at the global level has larger effects on local level responses than local level information has on global level responses (asymmetric interference effects). Traditionally, global shape primacy in Navon's paradigm has been linked with a right hemisphere preference (left visual field advantage) for global shape processing, and a left hemisphere preference (right visual field advantage) for local shape processing. This link, however, has been based on measures which confound global precedence and interference effects. Indeed, when these measures are de-confounded, we find no evidence for larger global precedence effects in the left compared with the right visual field in a large sample of participants (N = 337). In comparison, global-to-local interference effects are found to be stronger in the left than in the right visual field. We argue that these findings can be accounted for by assuming that the right hemisphere plays a special role in integrating shape information across spatial scales, that is, without assuming the existence of a right hemisphere preference for global shape processing per se.

Introduction

The idea that the cerebral hemispheres perform different functions was expressed already when Broca (1865) concluded that “We speak with the left hemisphere”. It took longer to appreciate the right hemisphere's modus operandi, and for many years it was simply considered minor or non-dominant (Ivry & Robertson, 1998). Even though the notion of major/minor hemisphere is abandoned today, the division of labor between the hemispheres is still debated. A functional difference that has caught on since it was first introduced in the 1960s (Levy-Agresti & Sperry, 1968) is the global/local (holistic/analytical) dichotomy according to which the right hemisphere (RH) is specialized for processing of global information, e.g. the overall layout of a scene, and the left hemisphere (LH) for processing of local information, e.g. details/parts.

A paradigm that has become central to studies of the global/local dichotomy is the one that Navon used in 1977 to answer the question: “Do we perceive a visual scene feature-by-feature? Or is the process instantaneous and simultaneous as some Gestalt psychologists believed?” (p. 353). Navon's paradigm is based on presentation of compound stimuli (Asch, 1962); usually a large letter (global level) composed of smaller letters (local level) in which the global and the local letters can be the same (consistent) or different (inconsistent) (see also Kinchla (1974) and Pomerantz and Sager (1975)). Navon made two observations with this paradigm which he argued supported “…the notion that global processing is a necessary stage of perception prior to more fine-grained analysis” (p. 371): (i) responses were faster to the global than the local level, and (ii) when the levels were inconsistent, information at the global level interfered with (slowed down) responses to the local level, but not the other way around.

Not long after, Martin (1979) used compound letters in a divided visual field paradigm to examine: (i) if global level responses were more efficient when stimuli were presented in the left visual field (VF) (RH) than when presented in the right VF (LH), and (ii) whether local level responses were more efficient when stimuli were presented in the right compared with the left VF. She found evidence for both propositions but argued that the right VF advantage for local level responses was stronger than the left VF advantage for global level responses. Martin's findings with normal subjects were soon confirmed in split-brain patients (Delis, Kramer, & Kiefner, 1988; Robertson, Lamb, & Zaidel, 1993), in patients with unilateral brain damage (Robertson & Lamb, 1991), and in functional imaging with normal subjects (Fink et al., 1996). Despite this convergence, VF differences in normal subjects have nevertheless proven rather flimsy being reported in some studies but not others (Blanca & Lopez-Montiel, 2009; Van Kleeck, 1989; Yovel, Levy, & Yovel, 2001), and it is still debated what might be the cause of these inconsistencies (Aiello et al., 2018; Flevaris & Robertson, 2016).

One obvious explanation may be that the VF effects captured with Navon's paradigm are small, and that failures to find them could reflect low statistical power (Lamb & Yund, 1996; Martens & Hübner, 2013). Given that most studies in this area are based on small samples (N < 30) this is not unlikely, and the magnitude of the effects when observed may thus also have been overestimated (Button et al., 2013; Open Science Collaboration, 2015). Another explanation is that different indexes can be derived from Navon's paradigm, and that these indexes do not necessarily measure the same aspects of global/local processing (Gerlach & Krumborg, 2014). Hence, there is ample evidence suggesting that what Navon originally referred to as global precedence actually reflects two independent effects: a global precedence effect and a global-to-local interference effect (Hübner & Volberg, 2005; Lamb & Yund, 1996; Robertson et al., 1993). More specifically, it has been suggested that the magnocellular pathway is responsible for the global precedence effect (Poirel et al., 2011; Poirel, Pineau, & Mellet, 2008), whereas the global-to-local interference effect rests upon automatic identification processes: Interference is observed as soon as a different identifiable stimulus is present at the global level during local level processing (Beaucousin et al., 2011). Consequently, during a global-local paradigm with compound letters such as the one used by Navon (1977), the identifiable distractor present at the global level is automatically processed and must be inhibited to perform correctly at the local level, leading to the global-to-local interference effect (Poirel et al., 2014).

Even though the global-local dichotomy has typically been associated with hemispheric differences, it has also been suggested that global information may be processed more efficiently in the lower than the upper VF whereas the reverse may be true for local information (Previc, 1990). Upper/lower VF differences have been found in many domains (Thomas & Elias, 2011), and may be linked to ventral and dorsal stream processing respectively (Milner & Goodale, 2006). Specific support for the notion that global/local processing may differ between the upper and lower VFs was reported by Christman (1993) who found a lower VF advantage for global level responses and an upper VF advantage for local level responses to inconsistent compound letters (experiment 1). Despite being larger than the left/right VF differences also found, the lower VF advantage for global level responses could not be replicated in his second experiment. Nevertheless, we concur with Christman (1993) “…that researchers studying left-right visual field differences in perceptual processing will need to address their relation to similar upper-lower visual field differences” (p. 278). Thus, the objective of the present study was to examine differences in global/local processing in all VFs (left/right & upper/lower) using compound letters. To address the shortcomings of previous studies, we examined a large sample (N = 337) by means of two indexes which tap global precedence and global-to-local interference effects reliably and independently. Based on the literature considered above both indexes were expected to yield higher values in the left than the right VF and in the lower compared with the upper VF.