Evidence of structure and persistence in motivational attraction to serial Pavlovian cues

Abstract

Sign-tracking is a form of autoshaping where animals develop conditioned responding directed toward stimuli predictive of an outcome even though the outcome is not contingent on the animal's behavior. Sign-tracking behaviors are thought to arise out of the attribution of incentive salience (i.e., motivational value) to reward-predictive cues. It is not known how incentive salience would be attributed to serially occurring cues, despite cues often occurring in a sequence in the real world as reward approaches. The experiments presented here demonstrate that reward-proximal cue responding is not altered by the presence of a distal reward cue (Experiment 1), and similarly that reward-distal cue responding which animals favor, is not altered by the presence of a reward-proximal cue (Experiment 2). Extinction of reward-proximal cues after training of the serial sequence leads to a generalized reduction in lever responding (Experiment 3). Together, we show that both Pavlovian serial lever cues acquire motivational value. These experiments also provide support to the notion that sign-tracking responses are insensitive to changes in outcome value, and that responding to serial cues creates a distinct context for outcome value.

Figure 1.

The presence of a distal lever does not alter proximal press rate responding throughout the 12 sessions of sign-tracking. Within Group Serial, a distal preference develops over time. (A) Timeline of experimental sessions for Group Serial (above) and Group Proximal (bottom). (Mag train) magazine training, (Ext) extinction test, (Reacq) reacquisition test, (Consum) consumption test in holding box. (B) Proximal press rates in presses per minute (ppm) in Group Serial (n = 8, white) and Group Proximal (n = 8, gray). Press rates were calculated by dividing overall presses within a session by total cue availability (250 sec) divided by 60 min. Error bars represent ±SEM. (C) Proximal (white) versus distal (black) press rates within Group Serial (n = 8) over 12 sessions of sign-tracking. (D) Difference scores of total proximal presses minus total distal presses were calculated for each animal within the serial group for each session and plotted as an average score per session. Lines projecting above the x-axis represent a positive score indicating proximal lever bias. (E) Difference scores of total proximal presses minus total distal presses were divided by the sum of all presses for each rat, for each session providing a number from −1 to 1. Scores were then split with 0 to 1 categorized as proximal bias for graphing purposes. Graph depicts percent of animals for each session with a proximal or distal bias. For example, session 5 depicts 75% (six out of eight of serial animals) with a distal bias. Error bars represent ±SEM.

Figure 2.

All animals are outcome insensitive in both extinction and reacquisition tests following devaluation. Consumption in holding boxes shows decrease in eating with successive LiCl pairings followed by increased eating in serial animals in post-devaluation reacquisition tests. (A) Press rates in presses per minute (ppm) on the proximal only group (n = 8, gray), proximal lever in the serial group (n = 8, white), and distal lever in the serial group (black) in extinction testing pre- and post-devaluation. Press rates were calculated by dividing overall presses within a session by total cue availability (∼60 sec) divided by total time (∼15 min). Error bars represent +SEM. (B) Press rates in presses per minute (ppm) in Group Proximal (gray), proximal lever in Group Serial (white), and distal lever in Group Serial (black) during reacquisition tests pre- and post-devaluation. Error bars represent +SEM. (C) Total grams consumed of pellets during devaluation sessions with holding box LiCl pairings in the Group Serial (white) and Group Proximal (gray) with error bars representing +SEM. The far right shows holding box consumption test conducted after the final reacquisition test. (D) Total pellets remaining in the chamber after reacquisition sessions in Group Serial (white) and Group Proximal (gray). Error bars represent +SEM.

Figure 3.

The presence of a proximal lever does not alter distal press rate responding throughout the 12 sessions of sign-tracking. Within serial animals, a distal preference develops over time. (A) Timeline of experimental sessions for Group Serial (above) and Group Distal (bottom). (B) Distal press rates in presses per minute (ppm) in Group Serial (n = 8, black) and Group Distal (n = 8, gray). Error bars represent ±SEM. (C) Proximal (white) versus distal (black) press rates within Group Serial (n = 8) over 12 sessions of training. Error bars represent ±SEM. (D) Difference scores of total proximal presses minus total distal presses were calculated for each animal within Group Serial for each session and plotted as an average score per session with error bars representing ±SEM. Lines projecting above the x-axis represent a positive score indicating proximal lever bias. (E) Difference scores of total proximal presses minus total distal presses were divided by the sum of all presses for each rat, for each session providing a number from −1 to 1. Scores were then split with 0 to 1 categorized as proximal bias for graphing purposes. Graph depicts percent of animals for each session with a proximal or distal bias (determined by difference scores).

Figure 4.

All animals are outcome insensitive in both extinction tests and reacquisition tests following devaluation. Consumption testing shows decrease in eating during LiCl pairings in holding box followed by increased eating in Group Serial in post-devaluation reacquisition tests. (A) Press rates in presses per minute (ppm) in Group Distal (n = 8, gray), proximal lever in Group Serial (n = 8, white), and distal lever in Group Serial (black) in extinction testing pre- and post-devaluation. Error bars represent ±SEM. (B) Press rates in presses per minute (ppm) in Group Distal (gray), proximal lever in Group Serial (white), and distal lever in Group Serial (black) in reacquisition tests pre- and post-devaluation. Error bars represent +SEM. (C) Total grams consumed of pellets during devaluation sessions with LiCl pairings in Group Serial (white) and Group Distal (gray) with error bars representing +SEM. The far right represents follow-up feeding tests conducted after the final reacquisition testing. (D) Total pellets remaining in the chamber after reacquisition sessions in Group Serial (white) and Group Distal (gray) indicates the larger bars as rejection of pellets. Error bars represent +SEM.

Figure 5.

Serial animals develop a distal preference over time. (A) Timeline of experimental sessions. All animals received the same magazine and serial sign-tracking programs and then animals were split into the Group Extinction with just the proximal lever presented (above) and Group Control with context exposure (bottom). (B) Proximal (white) versus distal (black) press rates over 12 training sessions (n = 16). Error bars represent ±SEM. (C) Difference scores of total proximal presses minus total distal presses were calculated for each animal for each session and plotted as an average score per session with error bars representing ±SEM. Lines projecting above the x-axis represent a positive score indicating proximal lever bias. (D) Difference scores of total proximal presses minus total distal presses were divided by the sum of all presses for each rat, for each session providing a number from −1 to 1. Scores were then split with 0 to 1 categorized as proximal bias for graphing purposes. Graph depicts percent of animals for each session with a proximal or distal bias. For example, session 3 depicts ∼63% (five out of eight of serial animals) with a distal bias.

Figure 6.

Animals given extinction on the proximal lever show reduced pressing compared to controls. All animals rapidly reacquire pressing on both levers. (A) Group Extinction (n = 8, white) versus Group Control (n = 8, black) average lever press rates in extinction test of serial sequence. Error bars represent +SEM. (B) Group Extinction (n = 8, solid line) versus Group Control (n = 8, dotted line) presses per trial over 25 trials in extinction. Proximal lever (white) presses per trial and distal presses per trial (black), each point represents the average of five trials, error bars represent +SEM. (C) Group Extinction (n = 8, white) versus Group Control (n = 8, black) average lever press rates in reacquisition test of serial sequence. Error bars represent +SEM. (D) Group Extinction (n = 8, solid line) versus Group Control (n = 8, dotted line) presses per trial over 25 trials in reacquisition. Proximal lever (white) presses per trial and distal presses per trial (black), each point represents the average of five trials, error bars represent +SEM.