5.1 Active sensing engages a world full of rhythms

Natural behaviour frequently involves the need to look for task-relevant information. The

function of attention is to facilitate this process by enhancing sensory signals from objects

that match our goals. Objects that are selected are more likely to enter into conscious

awareness, be remembered, and be acted upon.

– Yu et al., 2023

Objectives:

A. Learn how animals purposefully apply rhythmic motor movements to detect and

track biologically relevant sensory rhythms.

B. Apply knowledge of active sensing to real-world examples.

C. Understand how motor rhythms can enhance perception of anticipated events.

A theme in our previous chapter on perceptual learning was that motion factors large in a child’s early learning. We learned that the perception of motion will always betray the occurrence of an event, assuring the availability of instructive amodal properties. We saw how the desire to track moving objects trains smooth-pursuit eye movements. We discovered that a new motor plan provides a means to recognize when someone else is performing the same action. We also learned that biological motion provides social, emotional, and identity cues. And we learned that motion is the principal means by which we reveal affordances in the environment. Now, to begin our chapter on attention, we introduce another benefit of motion known as active sensing, the natural readiness of animals to gather useful information from the environment using rhythmic motor routines.

Inanimately generated rhythms in the environment tend to be large-scale rhythms that deeply resonate with us, such as when waves lap at a shoreline, fields of vegetation sway in the wind, or the sun cues daybreak or nightfall. On a moment-to-moment basis, however, environmental rhythms are more often generated by the animal kingdom. Rhythm detection is therefore a highly salient event, a near-certain signal that another animal is present. Imagine an animal spotting a distant motion in low-visibility conditions, say at dusk when the light is low. Even when vision cannot ascertain the figure of the animal causing the motion, a noted rhythm can betray a biological motion, such as gait, grooming behaviour, or tail movement. That rhythm would likely further suggest whether the detected animal is predator or prey or conspecific, fit or vulnerable or threatening, and in which direction it is facing or travelling. Those are impressive perceptual capabilities, but we would say they are receptive or passive.

Whereas the sights and sounds of biological rhythms can offer perceptual relevance to other animals, active sensing occurs when an animal produces rhythms of its own to enhance perception. More precisely, animals apply their motor and sensory systems in active and rhythmic ways to a situation. Many of these methods are familiar, and the variation across species is tremendous. Consider a few examples. Aroused by a vague scent, a gazelle shortens its regular breathing to a sniffing pattern. To clarify the texture of a surface, a mouse systematically whisks its whiskers. The “head drumming” behaviour of the blind mole rat against the roof of its underground tunnel creates seismic echoes that are picked up by sensors in its feet, allowing the detection and measurement of stones and other obstacles (Zweifel & Hartmann, 2020). These diverse examples illustrate two characteristics of active sensing. First, animals expend energy in rhythmic or quasi-rhythmic ways to enhance sensory information. Second, some species impose a rhythm where none had existed as a way of sampling the environment. Gathering these ideas, Schroeder et al. (2010) defined active sensing as a “collaboration of motor and sensory rhythms that is advantageous for information processing” (p. 172).

Leaving land, we find other extraordinary examples. Marine creatures often deal with murky or darker-than-night conditions in their search for food. To work around this problem, killer whales and other members of the Delphinidae (oceanic dolphin) family produce pulses of low- and high-frequency clicks which echo off potential prey to reveal their shape, size, speed, and direction of travel. Electric fishes emit an electric field and sense disruptions made by nearby prey. And harbour seals vibrate specialized whiskers at distinct frequencies to detect traces of turbulence left by a fish that might have passed several seconds ago. Similar adaptations are observed in air-travelling species. Some species of cave-dwelling birds produce audible clicks of their own to locate their roosts, and bats apply ultrasonic echolocation to navigate their environment and hunt airborne insects while avoiding collisions with one another.

Perhaps the most cited example of active sensing in humans relates to rhythmic eye

movements during visual search and is common to all primates (Schroeder et al., 2010;

Morillon, 2014). Rapid jumps of the eyes called “saccades” occur at the rate of about three per second, with each resulting in a brief fixation for information pickup. Because information pickup does not occur during the actual saccadic movement, this rhythmic motor process effectively organizes a pulsed visual signal (see footnote 1).

In comparison to vision, our sense of touch is better suited for active sensing. Purposeful

movement and touch each work to plan the way for the other. Humans derive some information when passively touched, but higher-quality information is obtained when the hands and fingers move in purposeful and often rhythmic ways to palm, pat, stroke, poke, or trace edges and surfaces (see footnote 2). These meaning-making methods are always exploratory and goal-oriented, and again reveal the inseparability of action and perception. When a small child pats or strokes a furry pet, an observer can usually note there is more to the behaviour than appeasing the animal. This is how a child explores texture, and we see the same rhythmic behaviours in use as a youngster acquires early experience with fabrics, foods, soil, and so on.

The importance of rhythm in active sensing is highlighted in the case of music perception. Our experience of music can involve rhythmic cues from multiple senses, but when we weave in a rhythmic motor routine, we are supporting precision timing, a talent overseen by the sense of hearing. The performing musician might tap a foot to keep time. An audience member might nod their head to the beat, and soon start finger-tapping as well. Someone else might get up to dance. In each of these examples, a primal urge to move initiates a rhythmic motor routine to find and fall in step with the song’s nested rhythms. The active involvement of motor routines in this manner is referred to as beat extraction, and it not only improves the perception of “on-beat targets” but suppresses the processing of potential distractions (Morillon et al., 2014; Morillon & Baillet, 2017).

Beyond music perception, body movement has been shown to support rhythm perception in both adults (Phillips-Silver & Trainor, 2007) and infants (Phillips-Silver & Trainor, 2005). The widely accepted explanation is that rhythms may be found in almost every situation, and initiating synchronizing rhythms of one’s own is the brain’s way of anticipating at which instants in time its resources will be most needed. That describes active sensing in its essence. Rhythm is in our nature, and motor rhythms help us track the rhythms of a situation to optimize sensory experience.

Primary school teachers routinely view young children applying rhythms to advantage their learning. For instance, many young students initially find it challenging to use a pair of scissors, and some find it helpful to move their tongue or jaw in a supportive rhythm. In a class of 6-year-olds, one can look around the room when scissors are in use and see a handful of students moving their oral apparatus in a strong rhythm with their working hand (see footnote 3). A more provocative example is witnessed when a class assembles on a carpet to listen to a story. The teacher of that same class of 6-year-olds is likely to see one or even two students rocking as the story is told, and on occasion a child will begin rocking before the story is underway. We might speculate that a rocking child is active sensing and is in some way making a special or necessary effort to pay attention by entraining to the storytelling experience. But would that mean there are rhythms in spoken language for active sensing to tune in to? What do you think?

Notes:

1. The small-scale motor movements involved are controlled by both voluntary and involuntary processes and are anticipatory in nature. With active sensing, then, we encounter another example of the close relationship between motor movement and perception. Once this action-perception process is underway, a system is formed, and there is little point in asking which is driving the other.

2. In childhood, touch experiences are especially rich because younger perceivers

have not yet learned to fully exploit vision as a partner in perceptual explorations.

Scheller (2019) reported that 5- to 7-year-olds show a strong reliance on active touch because, as discussed in Chapter 3, young children cannot integrate multiple sensory inputs at the optimal levels we observe in adulthood. At that age, touch will dominate vision in some situations, such as in size estimation (Broadbent et al., 2020).

3. Charles Darwin mentioned using scissors when discussing what he called the hand-mouth “sympathy” in his second-most famous book, The Expression of Emotions in Man and Animals (Darwin, 1872).