Is there a way to reference a top level field for preview?

I can see the issue in your code! When you want to reference the image field itself (which is the top-level field of your imageObject type), you should use asset to reference the image asset, not the schema type name.

In your current code, you have:

preview: {
  select: {
    title: 'caption',
    media: 'imageObject'  // This is incorrect
  }
}

The problem is that imageObject is the name of your schema type, not a field path. Since your type extends image (by setting type: 'image'), the actual image data is stored at the root level, and you need to reference the asset field specifically.

Here's the corrected version:

export default {
  name: 'imageObject',
  title: 'Image',
  type: 'image',
  fields: [
    { 
      name: 'caption',
      type: 'simpleBlockContent',
      title: 'Caption',
      options: {
        isHighlighted: true
      }
    }
  ],
  options: {
    hotspot: true
  },
  preview: {
    select: {
      title: 'caption',
      media: 'asset'  // Reference the image asset directly
    }
  }
}

According to the preview configuration documentation, when your type extends image, the asset field contains the reference to the image asset that the preview system needs to display the thumbnail. This should now properly show your image in the preview!